Fixes #19829.

Change-Id: I8500fd73c37b504d6ea25f5aff7017fbc0718570
Reviewed-on: https://go-review.googlesource.com/39314
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>