Fixes #13623.

Change-Id: I1bd96aa7b6b715e4dbdcf0c37c2d29228df6565c
Reviewed-on: https://go-review.googlesource.com/18329Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>