--- a/mercurial/unionrepo.py	Mon May 03 12:21:46 2021 +0200
+++ b/mercurial/unionrepo.py	Mon May 03 12:21:56 2021 +0200
@@ -174,7 +174,7 @@
         manifest.manifestrevlog.__init__(self, nodeconstants, opener)
         manifest2 = manifest.manifestrevlog(nodeconstants, opener2)
         unionrevlog.__init__(
-            self, opener, self.indexfile, manifest2, linkmapper
+            self, opener, self._revlog.indexfile, manifest2, linkmapper
         )