--- a/mercurial/discovery.py	Fri Jan 20 19:23:53 2012 +0100
+++ b/mercurial/discovery.py	Fri Jan 20 19:41:18 2012 +0100
@@ -124,8 +124,7 @@
                 missing.append(node)
         if excluded:
             # update missing heads
-            rset = repo.set('heads(%ln)', missing)
-            missingheads = [ctx.node() for ctx in rset]
+            missingheads = phases.newheads(repo, onlyheads, excluded)
         else:
             missingheads = onlyheads
         og.missingheads = missingheads