diff -r a3ce07e2dde5 -r 1814ca418b30 mercurial/branchmap.py
--- a/mercurial/branchmap.py	Thu Aug 10 18:46:55 2017 -0400
+++ b/mercurial/branchmap.py	Thu Aug 10 20:47:19 2017 -0700
@@ -406,7 +406,8 @@
 
         # 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