diff --git a/annif/backend/backend.py b/annif/backend/backend.py
index 6a5bbd7ad..44bbbb013 100644
--- a/annif/backend/backend.py
+++ b/annif/backend/backend.py
@@ -28,7 +28,7 @@ def __init__(self, backend_id, params, datadir):
     def fill_params_with_defaults(self):
         """Set the parameters that are not provided in the projects config file
         with default values defined in the backend class and its bases."""
-        for source_cls in self.__class__.mro()[:-1]:  # omit the object class
+        for source_cls in type(self).mro()[:-1]:  # omit the object class
             for default_param, default_value in \
                     source_cls.DEFAULT_PARAMS.items():
                 if default_param not in self.params: