diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1340,7 +1340,7 @@
             return True
 
         def maybewdir(prefix):
-            return all(c == 'f' for c in prefix)
+            return all(c == 'f' for c in pycompat.iterbytestr(prefix))
 
         hexnode = hex(node)