diff --git a/airflow/auth/managers/fab/fab_auth_manager.py b/airflow/auth/managers/fab/fab_auth_manager.py
index 52245090390748..bc480fd3a582bf 100644
--- a/airflow/auth/managers/fab/fab_auth_manager.py
+++ b/airflow/auth/managers/fab/fab_auth_manager.py
@@ -339,7 +339,7 @@ def security_manager(self) -> FabAirflowSecurityManagerOverride:
 
         sm_from_config = self.appbuilder.get_app.config.get("SECURITY_MANAGER_CLASS")
         if sm_from_config:
-            if not issubclass(sm_from_config, AirflowSecurityManager):
+            if issubclass(sm_from_config, AirflowSecurityManager):
                 raise Exception(
                     """Your CUSTOM_SECURITY_MANAGER must extend FabAirflowSecurityManagerOverride,
                      not FAB's own security manager."""