Fixes #12270.

Change-Id: Ie3dcbd0403d270b4b7f5c39049e12315eee159ed
Reviewed-on: https://go-review.googlesource.com/19837Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>