diff -r 9626022feaa4 -r d61c42c1a35c mercurial/revset.py
--- a/mercurial/revset.py	Sat Oct 15 17:10:53 2016 -0700
+++ b/mercurial/revset.py	Mon Oct 10 22:30:09 2016 +0200
@@ -1027,10 +1027,11 @@
         x = getstring(l[0], _("%s expected a pattern") % name)
         rev = None
         if len(l) >= 2:
-            rev = getset(repo, fullreposet(repo), l[1]).last()
-            if rev is None:
+            revs = getset(repo, fullreposet(repo), l[1])
+            if len(revs) != 1:
                 raise error.RepoLookupError(
-                        _("%s: starting revision set cannot be empty") % name)
+                        _("%s expected one starting revision") % name)
+            rev = revs.last()
             c = repo[rev]
         matcher = matchmod.match(repo.root, repo.getcwd(), [x],
                                  ctx=repo[rev], default='path')