Found by vet.

Change-Id: I1d78454facdd3522509ecfe7c73b21c4602ced8a
Reviewed-on: https://go-review.googlesource.com/32670
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>