Fixes #21219

Change-Id: I1a2ec1afe06586ed33a3a855b77536490cac3a38
Reviewed-on: https://go-review.googlesource.com/121115
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>