diff --git a/mercurial/unionrepo.py b/mercurial/unionrepo.py
--- a/mercurial/unionrepo.py
+++ b/mercurial/unionrepo.py
@@ -47,10 +47,13 @@
         self.repotiprev = n - 1
         self.bundlerevs = set() # used by 'bundle()' revset expression
         for rev2 in self.revlog2:
-            rev = self.revlog2.index[rev2]
-            # rev numbers - in revlog2, very different from self.rev
-            _start, _csize, _rsize, base, linkrev, p1rev, p2rev, node = rev
-            flags = _start & 0xFFFF
+            entry = self.revlog2.index[rev2]
+            flags = entry.offsetflags & 0xFFFF
+            base = entry.baserev
+            linkrev = entry.linkrev
+            p1rev = entry.p1rev
+            p2rev = entry.p2rev
+            node = entry.node
 
             if linkmapper is None: # link is to same revlog
                 assert linkrev == rev2 # we never link back