--- a/mercurial/unionrepo.py	Mon May 03 12:21:15 2021 +0200
+++ b/mercurial/unionrepo.py	Mon May 03 12:21:25 2021 +0200
@@ -97,6 +97,8 @@
                 self.rev(p1node),
                 self.rev(p2node),
                 node,
+                0,  # sidedata offset
+                0,  # sidedata size
             )
             self.index.append(e)
             self.bundlerevs.add(n)