Fixes #20898.

Change-Id: Ib3a8da34851d8b3681a6802e509fe712d6982df2
Reviewed-on: https://go-review.googlesource.com/47450
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>