Fixes #18181.

Change-Id: I5eed99dfb7b013aa4d4e668e95a97f5bb643d307
Reviewed-on: https://go-review.googlesource.com/34531
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>