diff --git a/hggit/gitrepo.py b/hggit/gitrepo.py
index cd32175a996b1bc4e8619b28632449339ae06b37_aGdnaXQvZ2l0cmVwby5weQ==..083d9683cb51e85cb26781872d6138dc3914de63_aGdnaXQvZ2l0cmVwby5weQ== 100644
--- a/hggit/gitrepo.py
+++ b/hggit/gitrepo.py
@@ -44,8 +44,8 @@
         return self.path
 
     def lookup(self, key):
-        if isinstance(key, bytes):
-            return key
+        assert isinstance(key, bytes)
+        return key
 
     def local(self):
         if not self.path: