Fixes #18911.

Change-Id: Ice10f37460a4f0a66cddeacfe26c28045f5e60fe
Reviewed-on: https://go-review.googlesource.com/116255
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>