diff --git a/stix2/core.py b/stix2/core.py
index c4d7628..b03e3d7 100644
--- a/stix2/core.py
+++ b/stix2/core.py
@@ -259,7 +259,7 @@ def _register_observable(new_observable, version=None):
 
 
 def _register_observable_extension(
-    observable, new_extension, version=stix2.DEFAULT_VERSION
+    observable, new_extension, version=stix2.DEFAULT_VERSION,
 ):
     """Register a custom extension to a STIX Cyber Observable type.