diff --git a/stix2/markings/object_markings.py b/stix2/markings/object_markings.py
index c0375c3..563d92b 100644
--- a/stix2/markings/object_markings.py
+++ b/stix2/markings/object_markings.py
@@ -37,7 +37,7 @@ def add_markings(obj, marking):
 
     object_markings = set(obj.get("object_marking_refs", []) + marking)
 
-    return new_version(obj, object_marking_refs=list(object_markings))
+    return new_version(obj, object_marking_refs=list(object_markings), allow_custom=True)
 
 
 def remove_markings(obj, marking):
diff --git a/stix2/test/test_custom.py b/stix2/test/test_custom.py
index 7c1832b..df0db79 100644
--- a/stix2/test/test_custom.py
+++ b/stix2/test/test_custom.py
@@ -2,7 +2,7 @@ import pytest
 
 import stix2
 
-from .constants import FAKE_TIME
+from .constants import FAKE_TIME, MARKING_DEFINITION_ID
 
 
 def test_identity_custom_property():
@@ -94,6 +94,25 @@ def test_custom_property_in_bundled_object():
     assert '"x_foo": "bar"' in str(bundle)
 
 
+def test_identity_custom_property_add_marking():
+    identity = stix2.Identity(
+        id="identity--311b2d2d-f010-5473-83ec-1edf84858f4c",
+        created="2015-12-21T19:59:11Z",
+        modified="2015-12-21T19:59:11Z",
+        name="John Smith",
+        identity_class="individual",
+        x_foo="bar",
+        allow_custom=True,
+    )
+    marking_definition = stix2.MarkingDefinition(
+        id=MARKING_DEFINITION_ID,
+        definition_type="statement",
+        definition=stix2.StatementMarking(statement="Copyright 2016, Example Corp")
+    )
+    identity2 = identity.add_markings(marking_definition)
+    assert identity2.x_foo == "bar"
+
+
 def test_custom_marking_no_init_1():
     @stix2.CustomMarking('x-new-obj', [
         ('property1', stix2.properties.StringProperty(required=True)),