diff --git a/mercurial/unionrepo.py b/mercurial/unionrepo.py
--- a/mercurial/unionrepo.py
+++ b/mercurial/unionrepo.py
@@ -49,7 +49,7 @@ class unionrevlog(revlog.revlog):
         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
+            _start, _csize, _rsize, base, linkrev, p1rev, p2rev, node = rev
 
             if linkmapper is None: # link is to same revlog
                 assert linkrev == rev2 # we never link back
@@ -57,6 +57,9 @@ class unionrevlog(revlog.revlog):
             else: # rev must be mapped from repo2 cl to unified cl by linkmapper
                 link = linkmapper(linkrev)
 
+            if linkmapper is not None: # link is to same revlog
+                base = linkmapper(base)
+
             if node in self.nodemap:
                 # this happens for the common revlog revisions
                 self.bundlerevs.add(self.nodemap[node])
@@ -65,7 +68,7 @@ class unionrevlog(revlog.revlog):
             p1node = self.revlog2.node(p1rev)
             p2node = self.revlog2.node(p2rev)
 
-            e = (None, None, None, None,
+            e = (None, None, None, base,
                  link, self.rev(p1node), self.rev(p2node), node)
             self.index.insert(-1, e)
             self.nodemap[node] = n