--- a/mercurial/branchmap.py	Sun Mar 12 16:44:01 2017 -0700
+++ b/mercurial/branchmap.py	Sun Mar 12 19:47:51 2017 -0400
@@ -408,7 +408,8 @@
 
         # fast path: extract data from cache, use it if node is matching
         reponode = changelog.node(rev)[:_rbcnodelen]
-        cachenode, branchidx = unpack_from(_rbcrecfmt, self._rbcrevs, rbcrevidx)
+        cachenode, branchidx = unpack_from(
+            _rbcrecfmt, util.buffer(self._rbcrevs), rbcrevidx)
         close = bool(branchidx & _rbccloseflag)
         if close:
             branchidx &= _rbcbranchidxmask