Fix backend

authens/backends.py

@@ -168,3 +168,10 @@ class OldCASBackend:
         # Taken from Django's ModelBackend
         is_active = getattr(user, "is_active", None)
         return is_active or is_active is None
+
+    # Django boilerplate.
+    def get_user(self, user_id):
+        try:
+            return UserModel.objects.get(pk=user_id)
+        except UserModel.DoesNotExist:
+            return None