diff --git a/python/triqs_tprf/ParameterCollection.py b/python/triqs_tprf/ParameterCollection.py
index e75085fe..5969b7a8 100644
--- a/python/triqs_tprf/ParameterCollection.py
+++ b/python/triqs_tprf/ParameterCollection.py
@@ -221,6 +221,8 @@ def getattr_from_objects(self, attr):
         return np.array([getattr(o, attr, None) for o in self.objects ])
     
     def __getattr__(self, attr):
+        if attr not in self.objects[0].keys():
+            raise AttributeError
         return self.getattr_from_objects(attr)
 
     def __reduce_to_dict__(self):