Fixes #21570

Change-Id: Ia0734929a04fbce8fdd5fbcb1b7baff9a8bbe39e
Reviewed-on: https://go-review.googlesource.com/58030Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>