diff --git a/stix2/v20/common.py b/stix2/v20/common.py
index 353ef70..ecc93b1 100644
--- a/stix2/v20/common.py
+++ b/stix2/v20/common.py
@@ -26,6 +26,7 @@ def _should_set_millisecond(cr, marking_type):
             return True
         return False
 
+
 class ExternalReference(_STIXBase):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709261>`__.
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index 881eced..4b13def 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -14,7 +14,6 @@ from ..properties import (
 )
 from ..utils import NOW, _get_dict
 
-
 class ExternalReference(_STIXBase):
     # TODO: Add link
     """For more detailed information on this object's properties, see