diff --git a/stix2/custom.py b/stix2/custom.py
index 0db9d25..802fd07 100644
--- a/stix2/custom.py
+++ b/stix2/custom.py
@@ -54,6 +54,9 @@ def _custom_marking_builder(cls, type, properties, version):
 
 
 def _custom_observable_builder(cls, type, properties, version, id_contrib_props=None):
+    if id_contrib_props is None:
+        id_contrib_props = []
+
     class _CustomObservable(cls, _Observable):
 
         if not re.match(TYPE_REGEX, type):
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 88d2f6c..e8c1925 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -980,9 +980,6 @@ def CustomObservable(type='x-custom-observable', properties=None, id_contrib_pro
         ...     pass
 
     """
-    if id_contrib_props is None:
-        id_contrib_props = []
-
     def wrapper(cls):
         _properties = list(itertools.chain.from_iterable([
             [('type', TypeProperty(type))],