Fixes #13011.

Change-Id: Ia4c67880fca83f4298ff6bb1b217ec26c8c83427
Reviewed-on: https://go-review.googlesource.com/17231Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>