Fixes #21615

Change-Id: Ic13190617d9b446b35f5dd00f142597c187ab669
Reviewed-on: https://go-review.googlesource.com/67270Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>