Fixes #7177.

Change-Id: Iba6063905f4f9c6acef8aba76b55d996f186d835
Reviewed-on: https://go-review.googlesource.com/29892Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>