Fixes #29837

Change-Id: I7d57c24d2133932c076df6f41dd6589f777b65dd
Reviewed-on: https://go-review.googlesource.com/c/158877
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>