Fixes #21763.

Change-Id: I59ee4f24c8064df64d9ede11aac02bc7ce4995b3
Reviewed-on: https://go-review.googlesource.com/61491Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>