diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
--- a/mercurial/changegroup.py
+++ b/mercurial/changegroup.py
@@ -733,7 +733,9 @@
 
         size = 0
         while tmfnodes:
-            dir = min(tmfnodes)
+            # Pick some element from tmfnodes, this is not necessarily the 'min'
+            # element.
+            dir = next(iter(tmfnodes))
             nodes = tmfnodes[dir]
             prunednodes = self.prune(dirlog(dir), nodes, commonrevs)
             if not dir or prunednodes: