Fixes #17623.

Change-Id: I4717e8399f955c9be7ba19108bb0bcc108187c04
Reviewed-on: https://go-review.googlesource.com/32147
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>