For #18130.
Fixes #18640.

Change-Id: I26cf1d1b78cca6ef207cc4333f30a9011ef347c9
Reviewed-on: https://go-review.googlesource.com/35831
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>