Fixes #21029

Change-Id: I308e2a2977870d8554a629f8ce38876598dba2a8
Reviewed-on: https://go-review.googlesource.com/48988
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>