diff --git a/mellon/backends.py b/mellon/backends.py
index 309ffb6..0e4726b 100644
--- a/mellon/backends.py
+++ b/mellon/backends.py
@@ -22,7 +22,7 @@ from . import utils
 
 class SAMLBackend(ModelBackend):
     def authenticate(self, request=None, **credentials):
-        saml_attributes = credentials.get('saml_attributes')
+        saml_attributes = credentials.get('saml_attributes') or {}
         # without an issuer we can do nothing
         if 'issuer' not in saml_attributes:
             return