diff --git a/tests/test_modeling_common.py b/tests/test_modeling_common.py
index e54cd5d8e..2e2db5b11 100755
--- a/tests/test_modeling_common.py
+++ b/tests/test_modeling_common.py
@@ -2339,7 +2339,7 @@ class ModelTesterMixin:
                 tied_params = [names for _, names in ptrs.items() if len(names) > 1]
                 for group in tied_params:
                     # We remove the group from extra_missing if not all weights from group are in it
-                    if len(group - extra_missing) > 0:
+                    if len(set(group) - extra_missing) > 0:
                         extra_missing = extra_missing - set(group)
 
                 self.assertEqual(