diff --git a/stix2/test/v20/test_custom.py b/stix2/test/v20/test_custom.py
index 57f8aac..33be3e2 100644
--- a/stix2/test/v20/test_custom.py
+++ b/stix2/test/v20/test_custom.py
@@ -288,6 +288,18 @@ def test_custom_marking_no_init_2():
     assert no2.property1 == 'something'
 
 
+def test_register_duplicate_marking():
+    with pytest.raises(DuplicateRegistrationError) as excinfo:
+        @stix2.v20.CustomMarking(
+            'x-new-obj2', [
+                ('property1', stix2.properties.StringProperty(required=True)),
+            ],
+        )
+        class NewObj2():
+            pass
+    assert "cannot be registered again" in str(excinfo.value)
+
+
 @stix2.v20.CustomObject(
     'x-new-type', [
         ('property1', stix2.properties.StringProperty(required=True)),
@@ -1119,15 +1131,3 @@ def test_register_duplicate_observable_extension():
         class NewExtension2():
             pass
     assert "cannot be registered again" in str(excinfo.value)
-
-
-def test_register_duplicate_marking():
-    with pytest.raises(DuplicateRegistrationError) as excinfo:
-        @stix2.v20.CustomMarking(
-            'x-new-obj-2', [
-                ('property1', stix2.properties.StringProperty(required=True)),
-            ],
-        )
-        class NewObj2():
-            pass
-    assert "cannot be registered again" in str(excinfo.value)
diff --git a/stix2/test/v21/test_custom.py b/stix2/test/v21/test_custom.py
index 3646f11..146abcd 100644
--- a/stix2/test/v21/test_custom.py
+++ b/stix2/test/v21/test_custom.py
@@ -397,6 +397,18 @@ def test_custom_marking_invalid_type_name():
             pass  # pragma: no cover
     assert "Invalid type name '7x-new-marking':" in str(excinfo.value)
 
+
+def test_register_duplicate_marking():
+    with pytest.raises(DuplicateRegistrationError) as excinfo:
+        @stix2.v21.CustomMarking(
+            'x-new-obj', [
+                ('property1', stix2.properties.StringProperty(required=True)),
+            ],
+        )
+        class NewObj2():
+            pass
+    assert "cannot be registered again" in str(excinfo.value)
+
 # Custom Objects
 
 
@@ -1326,15 +1338,3 @@ def test_register_duplicate_observable_extension():
         class NewExtension2():
             pass
     assert "cannot be registered again" in str(excinfo.value)
-
-
-def test_register_duplicate_marking():
-    with pytest.raises(DuplicateRegistrationError) as excinfo:
-        @stix2.v21.CustomMarking(
-            'x-new-obj', [
-                ('property1', stix2.properties.StringProperty(required=True)),
-            ],
-        )
-        class NewObj2():
-            pass
-    assert "cannot be registered again" in str(excinfo.value)