Fixes golang/go#18815.

Change-Id: Ic9d5cb640a555c58baedd597ed4ca5dd9f275c97
Reviewed-on: https://go-review.googlesource.com/36990
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>