--- a/mercurial/context.py	Sat Aug 20 22:37:58 2016 +0900
+++ b/mercurial/context.py	Fri Aug 19 18:40:35 2016 +0900
@@ -23,6 +23,7 @@
     short,
     wdirid,
     wdirnodes,
+    wdirrev,
 )
 from . import (
     encoding,
@@ -471,7 +472,7 @@
                 l = len(repo.changelog)
                 if r < 0:
                     r += l
-                if r < 0 or r >= l:
+                if r < 0 or r >= l and r != wdirrev:
                     raise ValueError
                 self._rev = r
                 self._node = repo.changelog.node(r)