Index: flexoentity/domain_manager.py
===================================================================
--- flexoentity/domain_manager.py	(revision 540da2b919b5e80fbffba37c244e0ac1ad784310)
+++ flexoentity/domain_manager.py	(revision a1c4ba3c9e7340534c8c5c9f5d8ad5613cbbe9c2)
@@ -32,4 +32,7 @@
 
     def __init__(self, local_backend, staging_backend, permanent_backend, registry=None):
+        assert local_backend is not None
+        assert staging_backend is not None
+        assert permanent_backend is not None
         super().__init__(local_backend=local_backend,
                          staging_backend=staging_backend,