Fixes #15820.

Change-Id: Ia5d5237754e77774a3a6049765eea163911f41c9
Reviewed-on: https://go-review.googlesource.com/25592Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/27054Reviewed-by: Chris Broadfoot <cbro@golang.org>