Found by vet. Introduced by CL 28853.

Change-Id: I3199e0cbdb1c512ba29eb7e4d5c1c98963f5a954
Reviewed-on: https://go-review.googlesource.com/28957
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>