--- a/mercurial/context.py	Thu Apr 18 16:46:09 2013 +0400
+++ b/mercurial/context.py	Thu Apr 18 19:50:04 2013 +0900
@@ -713,6 +713,7 @@
                         curr = pair(hist[p], curr)
                     if needed[p] == 1:
                         del hist[p]
+                        del needed[p]
                     else:
                         needed[p] -= 1