--- a/mercurial/debugcommands.py	Fri Jun 30 13:47:24 2017 +0200
+++ b/mercurial/debugcommands.py	Fri Jun 30 15:02:19 2017 +0200
@@ -2111,7 +2111,7 @@
     for rev in scmutil.revrange(repo, revs):
         ctx = repo[rev]
         ui.write('%s\n'% ctx2str(ctx))
-        for succsset in obsutil.successorssets(repo, ctx.node(), cache):
+        for succsset in obsutil.successorssets(repo, ctx.node(), cache=cache):
             if succsset:
                 ui.write('    ')
                 ui.write(node2str(succsset[0]))