Fixes golang/go#20558.

Change-Id: Iead39205e508aef4fa4cdbe5dabac96b0a60133a
Reviewed-on: https://go-review.googlesource.com/44775
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>