Fixes #13013

Change-Id: I6cf500eacdce76e303fc1cd92dd1c80eef0986bc
Reviewed-on: https://go-review.googlesource.com/16158Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>