Fixes #13635.

Change-Id: Icab4a45567f435f002a8f6c85db9538acf054a70
Reviewed-on: https://go-review.googlesource.com/17863
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>