Fixes #15179

Change-Id: I0f70b7ae1682eafaece7f22d8e76f0aa806f3ec9
Reviewed-on: https://go-review.googlesource.com/21589
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>