diff --git a/stix2/custom.py b/stix2/custom.py
index 81991e9..0db9d25 100644
--- a/stix2/custom.py
+++ b/stix2/custom.py
@@ -53,7 +53,7 @@ def _custom_marking_builder(cls, type, properties, version):
     return _CustomMarking
 
 
-def _custom_observable_builder(cls, type, properties, version, id_contrib_props=[]):
+def _custom_observable_builder(cls, type, properties, version, id_contrib_props=None):
     class _CustomObservable(cls, _Observable):
 
         if not re.match(TYPE_REGEX, type):
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 5c23362..88d2f6c 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -966,7 +966,7 @@ class X509Certificate(_Observable):
         self._check_at_least_one_property(att_list)
 
 
-def CustomObservable(type='x-custom-observable', properties=None, id_contrib_props=[]):
+def CustomObservable(type='x-custom-observable', properties=None, id_contrib_props=None):
     """Custom STIX Cyber Observable Object type decorator.
 
     Example:
@@ -980,6 +980,9 @@ 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))],