Fixes #12745

Change-Id: Iebb7c97cb5b68dc080644d796a6ca1c120d41b26
Reviewed-on: https://go-review.googlesource.com/27950Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>