diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index 317a2beb388..ae053f137e3 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -1026,7 +1026,7 @@ def __eq__(self, other):  # pragma: no cover
         assert lines is None
 
     def test_attrs_with_custom_eq(self) -> None:
-        @attr.define
+        @attr.define(slots=False)
         class SimpleDataObject:
             field_a = attr.ib()