Fixes #14219.

Change-Id: Id398dcfe6e9978d7eefddcdaaaa2256c16237cf3
Reviewed-on: https://go-review.googlesource.com/19207
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>