Fixes #28960

Change-Id: I0d049d4776dc42ef165a1da15f63de08677fbb85
Reviewed-on: https://go-review.googlesource.com/c/151661
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>