Fixes #18154.

Change-Id: I1836b63a380b7fc2f9965e909859b1a89a3ae5d5
Reviewed-on: https://go-review.googlesource.com/33856
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>