Fixes #18126.

Change-Id: I7ae090945ef203673b06eb94817cc5c894b5eadc
Reviewed-on: https://go-review.googlesource.com/33752
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>