mercurial/destutil.py

--- a/mercurial/destutil.py	Mon Jan 07 19:00:54 2019 -0500
+++ b/mercurial/destutil.py	Wed Jan 09 20:00:35 2019 -0800
@@ -381,6 +381,8 @@
         revs = stack.getstack(repo)
     elif default:
         revs = scmutil.revrange(repo, [default])
+    else:
+        raise error.Abort(_("config option histedit.defaultrev can't be empty"))
 
     if revs:
         # Take the first revision of the revset as the root