diff --git openid_client_ax.module openid_client_ax.module
index 4862590..091fd17 100644
--- openid_client_ax.module
+++ openid_client_ax.module
@@ -593,5 +608,7 @@ function openid_client_ax_schema_definitions() {
       'section'     => t('Extra-Information'),
     ),
   );
-  return $attributes;
+
+  // hook_openid_ax_schema()
+  return array_merge($attributes, module_invoke_all('openid_ax_schema'));
 }