diff --git a/src/saml2/assertion.py b/src/saml2/assertion.py
index 54d96172c..4c0ab1511 100644
--- a/src/saml2/assertion.py
+++ b/src/saml2/assertion.py
@@ -561,7 +561,7 @@ def restrict(self, ava, sp_entity_id, metadata=None):
         requirements_subject_id = metadata_store.subject_id_requirement(sp_entity_id) if metadata_store else []
         for r in requirements_subject_id:
             if r not in required_attributes:
-                required_attributes.extend(r)
+                required_attributes.append(r)
         return self.filter(
             ava,
             sp_entity_id,