Fixes #18673.

Change-Id: Ic827c16ad414733392c348da1c9ed9b308879fef
Reviewed-on: https://go-review.googlesource.com/35260
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>