diff --git a/tests/unittests/test_util.py b/tests/unittests/test_util.py
index ca15b4f9c91..176b4b3e40a 100644
--- a/tests/unittests/test_util.py
+++ b/tests/unittests/test_util.py
@@ -3372,7 +3372,7 @@ def test_logexc_with_log_level(self, caplog, log_level):
 
 
 class TestRandStr:
-    def _get_str_class_num(str):
+    def _get_str_class_num(self, str):
         import string
 
         str_class_num = 0