Fixes golang/go#17252.

Change-Id: I244c14d531542ea154f82e402059f4bc999241e2
Reviewed-on: https://go-review.googlesource.com/43211
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>