Fixes #14937.

Change-Id: Iea11a32230d44d5a43f8aec812d25f13bce85895
Reviewed-on: https://go-review.googlesource.com/21038Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>