Fixes #18933.

Change-Id: I8bb98e95bb4486a086d93bcf99e3a37488e77b03
Reviewed-on: https://go-review.googlesource.com/36318
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>