Fixes #20953

Change-Id: Ia30a6e0e335c1f738e1359500e09057b5981f1c7
Reviewed-on: https://go-review.googlesource.com/82397
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>