Fixes golang/go#16438.

Change-Id: I2a97e57cae298e8eecdd5637c9e03493a449fc62
Reviewed-on: https://go-review.googlesource.com/25070
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>