Found by vet.

Updates #11041

Change-Id: I5217b3e20c6af435d7500d6bb487b9895efe6605
Reviewed-on: https://go-review.googlesource.com/27493
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>