Fixes #13737.

Change-Id: Ib655dbf06f44709f687f8a2410c80f31e4075f13
Reviewed-on: https://go-review.googlesource.com/31322
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>