Fixes #16369.

Change-Id: I23f8c36370d0da37ac5b5126d012d22f78782782
Reviewed-on: https://go-review.googlesource.com/38392
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>