diff --git a/stix2/test/v21/test_course_of_action.py b/stix2/test/v21/test_course_of_action.py
index ce7f96f..5354d59 100644
--- a/stix2/test/v21/test_course_of_action.py
+++ b/stix2/test/v21/test_course_of_action.py
@@ -37,7 +37,7 @@ def test_course_of_action_example():
         description="This is how to add a filter rule to block inbound access to TCP port 80 to the existing UDP 1434 filter ...",
         action_type="textual:text/plain",
         os_execution_envs=["a", "b", "c"],
-        action_bin="aGVsbG8gd29ybGQ="
+        action_bin="aGVsbG8gd29ybGQ=",
     )
 
     assert str(coa) == EXPECTED
@@ -57,7 +57,7 @@ def test_course_of_action_example():
             "type": "course-of-action",
             "action_type": "textual:text/plain",
             "os_execution_envs": ["a", "b", "c"],
-            "action_bin": "aGVsbG8gd29ybGQ="
+            "action_bin": "aGVsbG8gd29ybGQ=",
         },
     ],
 )
@@ -84,8 +84,8 @@ def test_course_of_action_constraint():
             action_bin="aGVsbG8gd29ybGQ=",
             action_reference=stix2.v21.ExternalReference(
                 source_name="a source",
-                description="description of a source"
-            )
+                description="description of a source",
+            ),
         )
 
 # TODO: Add other examples
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index 2b1b49c..8e5cdc2 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -115,7 +115,7 @@ class CourseOfAction(STIXDomainObject):
 
         self._check_mutually_exclusive_properties(
             ["action_bin", "action_reference"],
-            at_least_one=False
+            at_least_one=False,
         )