Fixes bug referenced in this thread on golang-dev:
https://groups.google.com/d/topic/golang-dev/U4LSpMzL82c/discussion

Change-Id: If01a2644863f9e5625dd2f95f9d344bda772e12c
Reviewed-on: https://go-review.googlesource.com/12726Reviewed-by: Russ Cox <rsc@golang.org>