diff --git a/feeluown/library/library.py b/feeluown/library/library.py
index 09a532096f..ed02738a92 100644
--- a/feeluown/library/library.py
+++ b/feeluown/library/library.py
@@ -389,6 +389,8 @@ def _model_upgrade(self, model):
             elif e.reason is ModelNotFound.Reason.not_supported:
                 model.state = ModelState.cant_upgrade
             raise
+        if upgraded_model is None:  # some provider does not implement
+            raise ModelNotFound(f'{provider} implementation error, it returns None :(')
         return upgraded_model
 
     # --------
diff --git a/feeluown/library/provider.py b/feeluown/library/provider.py
index fb005ca14c..685583965e 100644
--- a/feeluown/library/provider.py
+++ b/feeluown/library/provider.py
@@ -41,6 +41,9 @@ def identifier(self):
     def name(self):
         """provider name"""
 
+    def __str__(self):
+        return f'provider:{self.identifier}'
+
     @contextmanager
     def auth_as(self, user):
         """auth as a user temporarily