Fixes golang/go#20558.

Change-Id: I8813d1eadd40680bd8619bb9a6539a3c45dcd76e
Reviewed-on: https://go-review.googlesource.com/44770
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>