--- a/mercurial/revset.py	Mon Mar 18 18:59:38 2019 +0300
+++ b/mercurial/revset.py	Mon Mar 18 19:01:29 2019 +0300
@@ -1240,7 +1240,7 @@
     getargs(x, 0, 0, _("head takes no arguments"))
     hs = set()
     cl = repo.changelog
-    for ls in repo.branchmap().itervalues():
+    for ls in repo.branchmap().iterheads():
         hs.update(cl.rev(h) for h in ls)
     return subset & baseset(hs)