Fixes golang/go#18809

Change-Id: Ib1014f3ebe5a57dde30b4eaf287a2cbff3c1179c
Reviewed-on: https://go-review.googlesource.com/36118
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>