Fixes #15577

Change-Id: I5f023790a393b17235db2e66c02c2483773ddc1a
Reviewed-on: https://go-review.googlesource.com/22857Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>