Fixes #13421.

Change-Id: Ic0fc0d0a8e86d2be23db12ddd1a8a8105e26ee56
Reviewed-on: https://go-review.googlesource.com/17250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>