diff --git a/docs/guide/datastore.ipynb b/docs/guide/datastore.ipynb
index 1c88cf1..1ea05ee 100644
--- a/docs/guide/datastore.ipynb
+++ b/docs/guide/datastore.ipynb
@@ -450,6 +450,14 @@
     "mem.source.filters.add([f1,f2])"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##### Note: The `defanged` property is now always included (implicitly) for STIX 2.1 Cyber Observable Objects (SCOs)\n",
+    "This is important to remember if you are writing a filter that involves checking the `objects` property of a STIX 2.1 `ObservedData` object. If any of the objects associated with the `objects` property are STIX 2.1 SCOs, then your filter must include the `defanged` property. For an example, refer to `filters[14]` & `filters[15]` in stix2/test/v21/test_datastore_filters.py "
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -726,21 +734,21 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "cti-python-stix2",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "cti-python-stix2"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
     "name": "ipython",
-    "version": 2
+    "version": 3
    },
    "file_extension": ".py",
    "mimetype": "text/x-python",
    "name": "python",
    "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython2",
-   "version": "2.7.12"
+   "pygments_lexer": "ipython3",
+   "version": "3.6.7"
   }
  },
  "nbformat": 4,
diff --git a/stix2/test/v21/test_observed_data.py b/stix2/test/v21/test_observed_data.py
index fbebdbd..0074bf7 100644
--- a/stix2/test/v21/test_observed_data.py
+++ b/stix2/test/v21/test_observed_data.py
@@ -465,21 +465,21 @@ def test_parse_email_message_not_multipart(data):
             "type": "file",
             "id": "file--65f2873d-38c2-56b4-bfa5-e3ef21e8a3c3",
             "hashes": {
-                "SHA-256": "19c549ec2628b989382f6b280cbd7bb836a0b461332c0fe53511ce7d584b89d3"
+                "SHA-1": "6e71b3cac15d32fe2d36c270887df9479c25c640"
             }
         },
         "2": {
             "type": "file",
             "id": "file--ef2d6dca-ec7d-5ab7-8dd9-ec9c0dee0eac",
             "hashes": {
-                "SHA-256": "0969de02ecf8a5f003e3f6d063d848c8a193aada092623f8ce408c15bcb5f038"
+                "SHA-512": "b7e98c78c24fb4c2c7b175e90474b21eae0ccf1b5ea4708b4e0f2d2940004419edc7161c18a1e71b2565df099ba017bcaa67a248e2989b6268ce078b88f2e210"
             }
         },
         "3": {
             "type": "file",
             "name": "foo.zip",
             "hashes": {
-                "SHA-256": "35a01331e9ad96f751278b891b6ea09699806faedfa237d40513d92ad1b7100f"
+                "SHA3-256": "35a01331e9ad96f751278b891b6ea09699806faedfa237d40513d92ad1b7100f"
             },
             "mime_type": "application/zip",
             "extensions": {
@@ -1490,3 +1490,39 @@ def test_deterministic_id_no_contributing_props():
     uuid_obj_2 = uuid.UUID(email_msg_2.id[-36:])
     assert uuid_obj_2.variant == uuid.RFC_4122
     assert uuid_obj_2.version == 4
+
+
+def test_ipv4_resolves_to_refs_deprecation():
+    with pytest.warns(stix2.exceptions.STIXDeprecationWarning):
+
+        stix2.v21.IPv4Address(
+            value="26.09.19.70",
+            resolves_to_refs=["mac-addr--08900593-0265-52fc-93c0-5b4a942f5887"],
+        )
+
+
+def test_ipv4_belongs_to_refs_deprecation():
+    with pytest.warns(stix2.exceptions.STIXDeprecationWarning):
+
+        stix2.v21.IPv4Address(
+            value="21.12.19.64",
+            belongs_to_refs=["autonomous-system--52e0a49d-d683-5801-a7b8-145765a1e116"],
+        )
+
+
+def test_ipv6_resolves_to_refs_deprecation():
+    with pytest.warns(stix2.exceptions.STIXDeprecationWarning):
+
+        stix2.v21.IPv6Address(
+            value="2001:0db8:85a3:0000:0000:8a2e:0370:7334",
+            resolves_to_refs=["mac-addr--08900593-0265-52fc-93c0-5b4a942f5887"],
+        )
+
+
+def test_ipv6_belongs_to_refs_deprecation():
+    with pytest.warns(stix2.exceptions.STIXDeprecationWarning):
+
+        stix2.v21.IPv6Address(
+            value="2001:0db8:85a3:0000:0000:8a2e:0370:7334",
+            belongs_to_refs=["autonomous-system--52e0a49d-d683-5801-a7b8-145765a1e116"],
+        )
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 5719e42..9fffeed 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(invalid_types="", 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')),
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index 4cae0e0..8bf76b8 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -6,11 +6,9 @@ import warnings
 
 from six.moves.urllib.parse import quote_plus
 
-from ..core import STIX2_OBJ_MAPS, STIXDomainObject
+from ..core import STIXDomainObject
 from ..custom import _custom_object_builder
-from ..exceptions import (
-    InvalidValueError, PropertyPresenceError, STIXDeprecationWarning,
-)
+from ..exceptions import PropertyPresenceError, STIXDeprecationWarning
 from ..properties import (
     BinaryProperty, BooleanProperty, EmbeddedObjectProperty, EnumProperty,
     FloatProperty, IDProperty, IntegerProperty, ListProperty,
@@ -597,16 +595,9 @@ class ObservedData(STIXDomainObject):
 
         self._check_mutually_exclusive_properties(
             ["objects", "object_refs"],
+            at_least_one=False,
         )
 
-        if self.get('object_refs'):
-            for identifier in self.get('object_refs'):
-                identifier_prefix = identifier[:identifier.index('--')]
-                if identifier_prefix in STIX2_OBJ_MAPS['v21']['observables'].keys():
-                    break
-            else:
-                raise InvalidValueError(self.__class__, 'object_refs', "At least one identifier must be of a SCO type if this property specified")
-
 
 class Opinion(STIXDomainObject):
     # TODO: Add link