--- a/mercurial/discovery.py	Fri Aug 26 12:48:20 2016 +0200
+++ b/mercurial/discovery.py	Tue Sep 06 09:43:25 2016 -0700
@@ -81,7 +81,7 @@
         # at least one of them must not be set
         assert None in (commonheads, missingroots)
         cl = repo.changelog
-        if not missingheads:
+        if missingheads is None:
             missingheads = cl.heads()
         if missingroots:
             discbases = []