diff --git a/mercurial/bundlerepo.py b/mercurial/bundlerepo.py
--- a/mercurial/bundlerepo.py
+++ b/mercurial/bundlerepo.py
@@ -99,7 +99,7 @@
         """return or calculate a delta between two revisions"""
         if rev1 > self.repotiprev and rev2 > self.repotiprev:
             # hot path for bundle
-            revb = self.index[rev2][3]
+            revb = self.index[rev2].baserev
             if revb == rev1:
                 return self._chunk(rev2)
         elif rev1 <= self.repotiprev and rev2 <= self.repotiprev:
@@ -131,7 +131,7 @@
                 rawtext = self._cache[2]
                 break
             chain.append(iterrev)
-            iterrev = self.index[iterrev][3]
+            iterrev = self.index[iterrev].baserev
         if rawtext is None:
             rawtext = self.baserevision(iterrev)