diff --git a/tests/unit/utils.py b/tests/unit/utils.py
index a7ab7c565..68d0fc33f 100644
--- a/tests/unit/utils.py
+++ b/tests/unit/utils.py
@@ -253,7 +253,7 @@ def _get_tester_for(self, column_type):
             return agate.TimeDelta()
 
         for instance in agate_helper.DEFAULT_TYPE_TESTER._possible_types:
-            if isinstance(instance, column_type):
+            if type(instance) is column_type:
                 return instance
 
         raise ValueError(f'no tester for {column_type}')