diff --git a/stix2/properties.py b/stix2/properties.py
index 0f68f6a..e63206a 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -14,7 +14,7 @@ import stix2.hashes
 from .base import _STIXBase
 from .exceptions import CustomContentError, DictionaryKeyError, STIXError
 from .parsing import parse, parse_observable
-from .registry import STIX2_OBJ_MAPS
+from .registry import class_for_type
 from .utils import (
     STIXTypeClass, _get_dict, get_class_hierarchy_names, get_type_from_id,
     is_object, is_stix_type, parse_into_datetime, to_enum,
@@ -781,17 +781,21 @@ class ExtensionsProperty(DictionaryProperty):
             raise ValueError("The extensions property must contain a dictionary")
 
         has_custom = False
-        extension_type_map = STIX2_OBJ_MAPS[self.spec_version].get('extensions', {})
         for key, subvalue in dictified.items():
-            if key in extension_type_map:
-                cls = extension_type_map[key]
-                if type(subvalue) is dict:
+            cls = class_for_type(key, self.spec_version, "extensions")
+            if cls:
+                if isinstance(subvalue, dict):
                     ext = cls(allow_custom=allow_custom, **subvalue)
-                elif type(subvalue) is cls:
-                    # If already an instance of an _Extension class, assume it's valid
+                elif isinstance(subvalue, cls):
+                    # If already an instance of the registered class, assume
+                    # it's valid
                     ext = subvalue
                 else:
-                    raise ValueError("Cannot determine extension type.")
+                    raise TypeError(
+                        "Can't create extension '{}' from {}.".format(
+                            key, type(subvalue)
+                        )
+                    )
 
                 has_custom = has_custom or ext.has_custom
 
@@ -805,15 +809,24 @@ class ExtensionsProperty(DictionaryProperty):
                 dictified[key] = ext
 
             else:
-                if allow_custom:
+                # If an unregistered "extension-definition--" style extension,
+                # we don't know what's supposed to be in it, so we can't
+                # determine whether there's anything custom.  So, assume there
+                # are no customizations.  If it's a different type of extension,
+                # non-registration implies customization (since all spec-defined
+                # extensions should be pre-registered with the library).
+
+                if key.startswith('extension-definition--'):
+                    _validate_id(
+                        key, self.spec_version, 'extension-definition--'
+                    )
+                elif allow_custom:
                     has_custom = True
-                    dictified[key] = subvalue
-                elif key.startswith('extension-definition--'):
-                    _validate_id(key, '2.1', 'extension-definition')
-                    dictified[key] = subvalue
                 else:
                     raise CustomContentError("Can't parse unknown extension type: {}".format(key))
 
+                dictified[key] = subvalue
+
         return dictified, has_custom
 
 
diff --git a/stix2/test/v20/test_custom.py b/stix2/test/v20/test_custom.py
index fd45ff4..4647167 100644
--- a/stix2/test/v20/test_custom.py
+++ b/stix2/test/v20/test_custom.py
@@ -795,7 +795,7 @@ def test_custom_extension_wrong_observable_type():
             },
         )
 
-    assert 'Cannot determine extension type' in excinfo.value.reason
+    assert "Can't create extension 'ntfs-ext' from" in excinfo.value.reason
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_custom.py b/stix2/test/v21/test_custom.py
index 4f60129..ff0464b 100644
--- a/stix2/test/v21/test_custom.py
+++ b/stix2/test/v21/test_custom.py
@@ -984,7 +984,7 @@ def test_custom_extension_wrong_observable_type():
             },
         )
 
-    assert 'Cannot determine extension type' in excinfo.value.reason
+    assert "Can't create extension 'ntfs-ext'" in excinfo.value.reason
 
 
 @pytest.mark.parametrize(
@@ -1228,6 +1228,41 @@ def test_parse_observable_with_unregistered_custom_extension(data):
     assert not isinstance(parsed_ob['extensions']['x-foobar-ext'], stix2.base._STIXBase)
 
 
+def test_unregistered_new_style_extension():
+
+    f_dict = {
+        "type": "file",
+        "name": "foo.txt",
+        "extensions": {
+            "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507": {
+                "extension-type": "property-extension",
+                "a": 1,
+                "b": True
+            }
+        }
+    }
+
+    f = stix2.parse(f_dict, allow_custom=False)
+
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["a"] == 1
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["b"]
+    assert not f.has_custom
+
+    f = stix2.parse(f_dict, allow_custom=True)
+
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["a"] == 1
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["b"]
+    assert not f.has_custom
+
+
 def test_register_custom_object():
     # Not the way to register custom object.
     class CustomObject2(object):