mercurial/branchmap.py

--- a/mercurial/branchmap.py	Wed Mar 15 23:28:39 2017 +0900
+++ b/mercurial/branchmap.py	Wed Mar 15 15:48:57 2017 -0700
@@ -452,6 +452,8 @@
 
     def _setcachedata(self, rev, node, branchidx):
         """Writes the node's branch data to the in-memory cache data."""
+        if rev == nullrev:
+            return
         rbcrevidx = rev * _rbcrecsize
         if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
             self._rbcrevs.extend('\0' *