diff --git a/stix2/core.py b/stix2/core.py
index 1031d61..aa9044d 100644
--- a/stix2/core.py
+++ b/stix2/core.py
@@ -8,7 +8,7 @@ import re
 import stix2
 
 from .base import _STIXBase
-from .exceptions import CustomContentError, ParseError
+from .exceptions import ParseError
 from .markings import _MarkingsMixin
 from .utils import _get_dict
 
@@ -109,7 +109,7 @@ def dict_to_stix2(stix_dict, allow_custom=False, version=None):
         # '2.0' representation.
         v = 'v20'
 
-    OBJ_MAP = STIX2_OBJ_MAPS[v]['objects']
+    OBJ_MAP = dict(STIX2_OBJ_MAPS[v]['objects'], **STIX2_OBJ_MAPS[v]['observables'])
 
     try:
         obj_class = OBJ_MAP[stix_dict['type']]
@@ -166,8 +166,8 @@ def parse_observable(data, _valid_refs=None, allow_custom=False, version=None):
             # flag allows for unknown custom objects too, but will not
             # be parsed into STIX observable object, just returned as is
             return obj
-        raise CustomContentError("Can't parse unknown observable type '%s'! For custom observables, "
-                                 "use the CustomObservable decorator." % obj['type'])
+        raise ParseError("Can't parse unknown observable type '%s'! For custom observables, "
+                         "use the CustomObservable decorator." % obj['type'])
 
     return obj_class(allow_custom=allow_custom, **obj)
 
diff --git a/stix2/custom.py b/stix2/custom.py
index 484cbb0..a00498b 100644
--- a/stix2/custom.py
+++ b/stix2/custom.py
@@ -67,19 +67,34 @@ def _custom_observable_builder(cls, type, properties, version):
         if not properties or not isinstance(properties, list):
             raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
 
-        # Check properties ending in "_ref/s" are ObjectReferenceProperties
-        for prop_name, prop in properties:
-            if prop_name.endswith('_ref') and ('ObjectReferenceProperty' not in get_class_hierarchy_names(prop)):
-                raise ValueError(
-                    "'%s' is named like an object reference property but "
-                    "is not an ObjectReferenceProperty." % prop_name,
-                )
-            elif (prop_name.endswith('_refs') and ('ListProperty' not in get_class_hierarchy_names(prop)
-                                                   or 'ObjectReferenceProperty' not in get_class_hierarchy_names(prop.contained))):
-                raise ValueError(
-                    "'%s' is named like an object reference list property but "
-                    "is not a ListProperty containing ObjectReferenceProperty." % prop_name,
-                )
+        if version == "2.0":
+            # If using STIX2.0, check properties ending in "_ref/s" are ObjectReferenceProperties
+            for prop_name, prop in properties:
+                if prop_name.endswith('_ref') and ('ObjectReferenceProperty' not in get_class_hierarchy_names(prop)):
+                    raise ValueError(
+                        "'%s' is named like an object reference property but "
+                        "is not an ObjectReferenceProperty." % prop_name,
+                    )
+                elif (prop_name.endswith('_refs') and ('ListProperty' not in get_class_hierarchy_names(prop) or
+                                                       'ObjectReferenceProperty' not in get_class_hierarchy_names(prop.contained))):
+                    raise ValueError(
+                        "'%s' is named like an object reference list property but "
+                        "is not a ListProperty containing ObjectReferenceProperty." % prop_name,
+                    )
+        else:
+            # If using STIX2.1 (or newer...), check properties ending in "_ref/s" are ReferenceProperties
+            for prop_name, prop in properties:
+                if prop_name.endswith('_ref') and ('ReferenceProperty' not in get_class_hierarchy_names(prop)):
+                    raise ValueError(
+                        "'%s' is named like a reference property but "
+                        "is not a ReferenceProperty." % prop_name,
+                    )
+                elif (prop_name.endswith('_refs') and ('ListProperty' not in get_class_hierarchy_names(prop) or
+                                                       'ReferenceProperty' not in get_class_hierarchy_names(prop.contained))):
+                    raise ValueError(
+                        "'%s' is named like a reference list property but "
+                        "is not a ListProperty containing ReferenceProperty." % prop_name,
+                    )
 
         _type = type
         _properties = OrderedDict(properties)
diff --git a/stix2/properties.py b/stix2/properties.py
index f6211c8..9ddfb82 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -454,22 +454,19 @@ class ReferenceProperty(Property):
             value = value.id
         value = str(value)
 
-        possible_prefix = value[:value.index('--') + 2]
+        possible_prefix = value[:value.index('--')]
 
         if self.valid_types:
-            if self.valid_types == ["only_SDO"]:
-                self.valid_types = STIX2_OBJ_MAPS['v21']['objects'].keys()
-            elif self.valid_types == ["only_SCO"]:
-                self.valid_types = STIX2_OBJ_MAPS['v21']['observables'].keys()
-            elif self.valid_types == ["only_SCO_&_SRO"]:
-                self.valid_types = list(STIX2_OBJ_MAPS['v21']['observables'].keys()) + ['relationship', 'sighting']
+            ref_valid_types = enumerate_types(self.valid_types, 'v' + self.spec_version.replace(".", ""))
 
-            if possible_prefix[:-2] in self.valid_types:
+            if possible_prefix in ref_valid_types:
                 required_prefix = possible_prefix
             else:
                 raise ValueError("The type-specifying prefix '%s' for this property is not valid" % (possible_prefix))
         elif self.invalid_types:
-            if possible_prefix[:-2] not in self.invalid_types:
+            ref_invalid_types = enumerate_types(self.invalid_types, 'v' + self.spec_version.replace(".", ""))
+
+            if possible_prefix not in ref_invalid_types:
                 required_prefix = possible_prefix
             else:
                 raise ValueError("An invalid type-specifying prefix '%s' was specified for this property" % (possible_prefix, value))
@@ -479,6 +476,31 @@ class ReferenceProperty(Property):
         return value
 
 
+def enumerate_types(types, spec_version):
+    """
+    `types` is meant to be a list; it may contain specific object types and/or
+        the any of the words "SCO", "SDO", or "SRO"
+
+    Since "SCO", "SDO", and "SRO" are general types that encompass various specific object types,
+        once each of those words is being processed, that word will be removed from `return_types`,
+        so as not to mistakenly allow objects to be created of types "SCO", "SDO", or "SRO"
+    """
+    return_types = []
+    return_types += types
+
+    if "SDO" in types:
+        return_types.remove("SDO")
+        return_types += STIX2_OBJ_MAPS[spec_version]['objects'].keys()
+    if "SCO" in types:
+        return_types.remove("SCO")
+        return_types += STIX2_OBJ_MAPS[spec_version]['observables'].keys()
+    if "SRO" in types:
+        return_types.remove("SRO")
+        return_types += ['relationship', 'sighting']
+
+    return return_types
+
+
 SELECTOR_REGEX = re.compile(r"^[a-z0-9_-]{3,250}(\.(\[\d+\]|[a-z0-9_-]{1,250}))*$")
 
 
diff --git a/stix2/test/v20/test_custom.py b/stix2/test/v20/test_custom.py
index 6d127f2..ce1aac3 100644
--- a/stix2/test/v20/test_custom.py
+++ b/stix2/test/v20/test_custom.py
@@ -583,7 +583,7 @@ def test_parse_unregistered_custom_observable_object():
         "property1": "something"
     }"""
 
-    with pytest.raises(stix2.exceptions.CustomContentError) as excinfo:
+    with pytest.raises(stix2.exceptions.ParseError) as excinfo:
         stix2.parse_observable(nt_string, version='2.0')
     assert "Can't parse unknown observable type" in str(excinfo.value)
 
diff --git a/stix2/test/v21/test_base.py b/stix2/test/v21/test_base.py
index d753ab1..325cf4b 100644
--- a/stix2/test/v21/test_base.py
+++ b/stix2/test/v21/test_base.py
@@ -29,7 +29,7 @@ def test_encode_json_object():
 
 def test_deterministic_id_unicode():
     mutex = {'name': u'D*Fl#Ed*\u00a3\u00a8', 'type': 'mutex'}
-    obs = stix2.parse_observable(mutex, version="2.1")
+    obs = stix2.parse(mutex, version="2.1")
 
     dd_idx = obs.id.index("--")
     id_uuid = uuid.UUID(obs.id[dd_idx+2:])
diff --git a/stix2/test/v21/test_custom.py b/stix2/test/v21/test_custom.py
index 47d484a..9c650eb 100644
--- a/stix2/test/v21/test_custom.py
+++ b/stix2/test/v21/test_custom.py
@@ -508,7 +508,7 @@ def test_custom_observable_object_invalid_ref_property():
         )
         class NewObs():
             pass
-    assert "is named like an object reference property but is not an ObjectReferenceProperty" in str(excinfo.value)
+    assert "is named like a reference property but is not a ReferenceProperty" in str(excinfo.value)
 
 
 def test_custom_observable_object_invalid_refs_property():
@@ -520,7 +520,7 @@ def test_custom_observable_object_invalid_refs_property():
         )
         class NewObs():
             pass
-    assert "is named like an object reference list property but is not a ListProperty containing ObjectReferenceProperty" in str(excinfo.value)
+    assert "is named like a reference list property but is not a ListProperty containing ReferenceProperty" in str(excinfo.value)
 
 
 def test_custom_observable_object_invalid_refs_list_property():
@@ -532,26 +532,7 @@ def test_custom_observable_object_invalid_refs_list_property():
         )
         class NewObs():
             pass
-    assert "is named like an object reference list property but is not a ListProperty containing ObjectReferenceProperty" in str(excinfo.value)
-
-
-def test_custom_observable_object_invalid_valid_refs():
-    @stix2.v21.CustomObservable(
-        'x-new-obs', [
-            ('property1', stix2.properties.StringProperty(required=True)),
-            ('property_ref', stix2.properties.ObjectReferenceProperty(valid_types='email-addr')),
-        ],
-    )
-    class NewObs():
-        pass
-
-    with pytest.raises(Exception) as excinfo:
-        NewObs(
-            _valid_refs=['1'],
-            property1='something',
-            property_ref='1',
-        )
-    assert "must be created with _valid_refs as a dict, not a list" in str(excinfo.value)
+    assert "is named like a reference list property but is not a ListProperty containing ReferenceProperty" in str(excinfo.value)
 
 
 def test_custom_no_properties_raises_exception():
@@ -575,8 +556,7 @@ def test_parse_custom_observable_object():
         "type": "x-new-observable",
         "property1": "something"
     }"""
-
-    nt = stix2.parse_observable(nt_string, [], version='2.1')
+    nt = stix2.parse(nt_string, [], version='2.1')
     assert isinstance(nt, stix2.base._STIXBase)
     assert nt.property1 == 'something'
 
@@ -587,11 +567,10 @@ def test_parse_unregistered_custom_observable_object():
         "property1": "something"
     }"""
 
-    with pytest.raises(stix2.exceptions.CustomContentError) as excinfo:
-        stix2.parse_observable(nt_string, version='2.1')
-    assert "Can't parse unknown observable type" in str(excinfo.value)
-
-    parsed_custom = stix2.parse_observable(nt_string, allow_custom=True, version='2.1')
+    with pytest.raises(stix2.exceptions.ParseError) as excinfo:
+        stix2.parse(nt_string, version='2.1')
+    assert "Can't parse unknown object type" in str(excinfo.value)
+    parsed_custom = stix2.parse(nt_string, allow_custom=True, version='2.1')
     assert parsed_custom['property1'] == 'something'
     with pytest.raises(AttributeError) as excinfo:
         assert parsed_custom.property1 == 'something'
@@ -604,8 +583,8 @@ def test_parse_unregistered_custom_observable_object_with_no_type():
     }"""
 
     with pytest.raises(stix2.exceptions.ParseError) as excinfo:
-        stix2.parse_observable(nt_string, allow_custom=True, version='2.1')
-    assert "Can't parse observable with no 'type' property" in str(excinfo.value)
+        stix2.parse(nt_string, allow_custom=True, version='2.1')
+    assert "Can't parse object with no 'type' property" in str(excinfo.value)
 
 
 def test_parse_observed_data_with_custom_observable():
@@ -634,8 +613,8 @@ def test_parse_invalid_custom_observable_object():
     }"""
 
     with pytest.raises(stix2.exceptions.ParseError) as excinfo:
-        stix2.parse_observable(nt_string, version='2.1')
-    assert "Can't parse observable with no 'type' property" in str(excinfo.value)
+        stix2.parse(nt_string, version='2.1')
+    assert "Can't parse object with no 'type' property" in str(excinfo.value)
 
 
 def test_observable_custom_property():
@@ -885,8 +864,7 @@ def test_parse_observable_with_custom_extension():
             }
         }
     }"""
-
-    parsed = stix2.parse_observable(input_str, version='2.1')
+    parsed = stix2.parse(input_str, version='2.1')
     assert parsed.extensions['x-new-ext'].property2 == 12
 
 
@@ -961,10 +939,9 @@ def test_custom_and_spec_extension_mix():
 )
 def test_parse_observable_with_unregistered_custom_extension(data):
     with pytest.raises(InvalidValueError) as excinfo:
-        stix2.parse_observable(data, version='2.1')
+        stix2.parse(data, version='2.1')
     assert "Can't parse unknown extension type" in str(excinfo.value)
-
-    parsed_ob = stix2.parse_observable(data, allow_custom=True, version='2.1')
+    parsed_ob = stix2.parse(data, allow_custom=True, version='2.1')
     assert parsed_ob['extensions']['x-foobar-ext']['property1'] == 'foo'
     assert not isinstance(parsed_ob['extensions']['x-foobar-ext'], stix2.base._STIXBase)
 
diff --git a/stix2/test/v21/test_observed_data.py b/stix2/test/v21/test_observed_data.py
index 2910ba1..32bd0bf 100644
--- a/stix2/test/v21/test_observed_data.py
+++ b/stix2/test/v21/test_observed_data.py
@@ -209,7 +209,7 @@ def test_observed_data_example_with_bad_refs():
 
     assert excinfo.value.cls == stix2.v21.Directory
     assert excinfo.value.prop_name == "contains_refs"
-    assert "The type-specifying prefix 'monkey--' for this property is not valid" in excinfo.value.reason
+    assert "The type-specifying prefix 'monkey' for this property is not valid" in excinfo.value.reason
 
 
 def test_observed_data_example_with_non_dictionary():
@@ -369,7 +369,7 @@ def test_parse_autonomous_system_valid(data):
     ],
 )
 def test_parse_email_address(data):
-    odata = stix2.parse_observable(data, version='2.1')
+    odata = stix2.parse(data, version='2.1')
     assert odata.type == "email-addr"
 
     odata_str = re.compile(
@@ -378,7 +378,7 @@ def test_parse_email_address(data):
         '"belongs_to_ref": "mutex--9be6365f-b89c-48c0-9340-6953f6595718"', data,
     )
     with pytest.raises(stix2.exceptions.InvalidValueError):
-        stix2.parse_observable(odata_str, version='2.1')
+        stix2.parse(odata_str, version='2.1')
 
 
 @pytest.mark.parametrize(
@@ -424,7 +424,7 @@ def test_parse_email_address(data):
     ],
 )
 def test_parse_email_message(data):
-    odata = stix2.parse_observable(data, version='2.1')
+    odata = stix2.parse(data, version='2.1')
     assert odata.type == "email-message"
     assert odata.body_multipart[0].content_disposition == "inline"
 
@@ -446,7 +446,7 @@ def test_parse_email_message(data):
 )
 def test_parse_email_message_not_multipart(data):
     with pytest.raises(stix2.exceptions.DependentPropertiesError) as excinfo:
-        stix2.parse_observable(data, version='2.1')
+        stix2.parse(data, version='2.1')
 
     assert excinfo.value.cls == stix2.v21.EmailMessage
     assert excinfo.value.dependencies == [("is_multipart", "body")]
@@ -548,7 +548,7 @@ def test_parse_file_archive(data):
 )
 def test_parse_email_message_with_at_least_one_error(data):
     with pytest.raises(stix2.exceptions.InvalidValueError) as excinfo:
-        stix2.parse_observable(data, version='2.1')
+        stix2.parse(data, version='2.1')
 
     assert excinfo.value.cls == stix2.v21.EmailMessage
     assert "At least one of the" in str(excinfo.value)
@@ -570,7 +570,7 @@ def test_parse_email_message_with_at_least_one_error(data):
     ],
 )
 def test_parse_basic_tcp_traffic(data):
-    odata = stix2.parse_observable(
+    odata = stix2.parse(
         data, version='2.1',
     )
 
@@ -602,7 +602,7 @@ def test_parse_basic_tcp_traffic(data):
 )
 def test_parse_basic_tcp_traffic_with_error(data):
     with pytest.raises(stix2.exceptions.AtLeastOnePropertyError) as excinfo:
-        stix2.parse_observable(data, version='2.1')
+        stix2.parse(data, version='2.1')
 
     assert excinfo.value.cls == stix2.v21.NetworkTraffic
     assert excinfo.value.properties == ["dst_ref", "src_ref"]
diff --git a/stix2/v20/sdo.py b/stix2/v20/sdo.py
index 19c53d8..9ba1860 100644
--- a/stix2/v20/sdo.py
+++ b/stix2/v20/sdo.py
@@ -233,7 +233,7 @@ class Report(STIXDomainObject):
         ('name', StringProperty(required=True)),
         ('description', StringProperty()),
         ('published', TimestampProperty(required=True)),
-        ('object_refs', ListProperty(ReferenceProperty(invalid_types=[""], spec_version='2.0'), required=True)),
+        ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.0'), required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty, required=True)),
         ('external_references', ListProperty(ExternalReference)),
diff --git a/stix2/v20/sro.py b/stix2/v20/sro.py
index 3f561b8..b85eb68 100644
--- a/stix2/v20/sro.py
+++ b/stix2/v20/sro.py
@@ -67,7 +67,7 @@ class Sighting(STIXRelationshipObject):
         ('first_seen', TimestampProperty()),
         ('last_seen', TimestampProperty()),
         ('count', IntegerProperty(min=0, max=999999999)),
-        ('sighting_of_ref', ReferenceProperty(valid_types="only_SDO", spec_version='2.0', required=True)),
+        ('sighting_of_ref', ReferenceProperty(valid_types="SDO", spec_version='2.0', required=True)),
         ('observed_data_refs', ListProperty(ReferenceProperty(valid_types='observed-data', spec_version='2.0'))),
         ('where_sighted_refs', ListProperty(ReferenceProperty(valid_types='identity', spec_version='2.0'))),
         ('summary', BooleanProperty(default=lambda: False)),
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index a31e710..4a71308 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -76,7 +76,7 @@ class LanguageContent(_STIXBase):
         ('created_by_ref', ReferenceProperty(valid_types='identity', spec_version='2.1')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
-        ('object_ref', ReferenceProperty(invalid_types=[""], spec_version='2.1', required=True)),
+        ('object_ref', ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1', required=True)),
         # TODO: 'object_modified' it MUST be an exact match for the modified time of the STIX Object (SRO or SDO) being referenced.
         ('object_modified', TimestampProperty(precision='millisecond')),
         # TODO: 'contents' https://docs.google.com/document/d/1ShNq4c3e1CkfANmD9O--mdZ5H0O_GLnjN28a_yrEaco/edit#heading=h.cfz5hcantmvx
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index fdb61f4..b08869e 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -385,7 +385,7 @@ class File(_Observable):
         ('mtime', TimestampProperty()),
         ('atime', TimestampProperty()),
         ('parent_directory_ref', ReferenceProperty(valid_types='directory', spec_version='2.1')),
-        ('contains_refs', ListProperty(ReferenceProperty(invalid_types="", spec_version='2.1'))),
+        ('contains_refs', ListProperty(ReferenceProperty(valid_types=["SCO"], spec_version='2.1'))),
         ('content_ref', ReferenceProperty(valid_types='artifact', spec_version='2.1')),
         ('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=_type)),
         ('spec_version', StringProperty(fixed='2.1')),
@@ -1028,6 +1028,7 @@ def CustomObservable(type='x-custom-observable', properties=None):
     def wrapper(cls):
         _properties = list(itertools.chain.from_iterable([
             [('type', TypeProperty(type))],
+            [('id', IDProperty(type, spec_version='2.1'))],
             properties,
             [('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=type))],
         ]))
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index 6f7b52d..db45ed6 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -149,7 +149,7 @@ class Grouping(STIXDomainObject):
         ('name', StringProperty()),
         ('description', StringProperty()),
         ('context', StringProperty(required=True)),
-        ('object_refs', ListProperty(ReferenceProperty(invalid_types=[""], spec_version='2.1'), required=True)),
+        ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1'), required=True)),
     ])
 
 
@@ -512,7 +512,7 @@ class MalwareAnalysis(STIXDomainObject):
         ('analysis_started', TimestampProperty()),
         ('analysis_ended', TimestampProperty()),
         ('av_result', StringProperty()),
-        ('analysis_sco_refs', ListProperty(ReferenceProperty(valid_types="only_SCO", spec_version='2.1'))),
+        ('analysis_sco_refs', ListProperty(ReferenceProperty(valid_types="SCO", spec_version='2.1'))),
     ])
 
     def _check_object_constraints(self):
@@ -538,7 +538,7 @@ class Note(STIXDomainObject):
         ('abstract', StringProperty()),
         ('content', StringProperty(required=True)),
         ('authors', ListProperty(StringProperty)),
-        ('object_refs', ListProperty(ReferenceProperty(invalid_types=[""], spec_version='2.1'), required=True)),
+        ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1'), required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('confidence', IntegerProperty()),
@@ -567,7 +567,7 @@ class ObservedData(STIXDomainObject):
         ('last_observed', TimestampProperty(required=True)),
         ('number_observed', IntegerProperty(min=1, max=999999999, required=True)),
         ('objects', ObservableProperty(spec_version='2.1')),
-        ('object_refs', ListProperty(ReferenceProperty(valid_types="only_SCO_&_SRO", spec_version="2.1"))),
+        ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SRO"], spec_version="2.1"))),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('confidence', IntegerProperty()),
@@ -632,7 +632,7 @@ class Opinion(STIXDomainObject):
                 ], required=True,
             ),
         ),
-        ('object_refs', ListProperty(ReferenceProperty(invalid_types=[""], spec_version='2.1'), required=True)),
+        ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1'), required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('confidence', IntegerProperty()),
@@ -661,7 +661,7 @@ class Report(STIXDomainObject):
         ('description', StringProperty()),
         ('report_types', ListProperty(StringProperty, required=True)),
         ('published', TimestampProperty(required=True)),
-        ('object_refs', ListProperty(ReferenceProperty(invalid_types=[""], spec_version='2.1'), required=True)),
+        ('object_refs', ListProperty(ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1'), required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('confidence', IntegerProperty()),
diff --git a/stix2/v21/sro.py b/stix2/v21/sro.py
index 57c7719..059bb66 100644
--- a/stix2/v21/sro.py
+++ b/stix2/v21/sro.py
@@ -86,7 +86,7 @@ class Sighting(STIXRelationshipObject):
         ('first_seen', TimestampProperty()),
         ('last_seen', TimestampProperty()),
         ('count', IntegerProperty(min=0, max=999999999)),
-        ('sighting_of_ref', ReferenceProperty(valid_types="only_SDO", spec_version='2.1', required=True)),
+        ('sighting_of_ref', ReferenceProperty(valid_types="SDO", spec_version='2.1', required=True)),
         ('observed_data_refs', ListProperty(ReferenceProperty(valid_types='observed-data', spec_version='2.1'))),
         ('where_sighted_refs', ListProperty(ReferenceProperty(valid_types='identity', spec_version='2.1'))),
         ('summary', BooleanProperty()),