diff --git a/tests/test_ujson.py b/tests/test_ujson.py
index f4a5e447..e722ab50 100644
--- a/tests/test_ujson.py
+++ b/tests/test_ujson.py
@@ -831,7 +831,7 @@ def test_encode_none_key():
 
 
 def test_default_function():
-    iso8601_time_format = '%Y-%m-%dT%H:%M:%S.%f'
+    iso8601_time_format = "%Y-%m-%dT%H:%M:%S.%f"
 
     def default(value):
         if isinstance(value, datetime.datetime):