diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -406,8 +406,7 @@ class revbranchcache(object):
 
         # fast path: extract data from cache, use it if node is matching
         reponode = changelog.node(rev)[:_rbcnodelen]
-        cachenode, branchidx = unpack_from(
-            _rbcrecfmt, util.buffer(self._rbcrevs), rbcrevidx)
+        cachenode, branchidx = unpack_from(_rbcrecfmt, self._rbcrevs, rbcrevidx)
         close = bool(branchidx & _rbccloseflag)
         if close:
             branchidx &= _rbcbranchidxmask