Fixes #11626.

Change-Id: I1b70c0844473c3b57a53d7cca747ea5cdc68d232
Reviewed-on: https://go-review.googlesource.com/14526
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>