diff -r b0c73866c9fb -r 5b32b3c618b2 mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Thu Aug 16 19:23:24 2018 +0000
+++ b/mercurial/debugcommands.py	Thu Aug 16 19:39:47 2018 +0000
@@ -791,9 +791,11 @@
             if not opts.get('nonheads'):
                 ui.write(("unpruned common: %s\n") %
                          " ".join(sorted(short(n) for n in common)))
-                dag = dagutil.revlogdag(repo.changelog)
+                cl = repo.changelog
+                clnode = cl.node
+                dag = dagutil.revlogdag(cl)
                 all = dag.ancestorset(dag.internalizeall(common))
-                common = dag.externalizeall(dag.headsetofconnecteds(all))
+                common = {clnode(r) for r in dag.headsetofconnecteds(all)}
         else:
             nodes = None
             if pushedrevs: