Fixes #6879.

Change-Id: I9ed2460cf14cb9322d9521e7af910efa48abdaf0
Reviewed-on: https://go-review.googlesource.com/23112
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>