Fixes golang/go#15999.

Change-Id: I20793ce717c768557c4942ff6be4e77c23ab201c
Reviewed-on: https://go-review.googlesource.com/23880
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>