Fixes #21343

Change-Id: I3582fced902592fe12bfa29acf7b40b6e5e554a7
Reviewed-on: https://go-review.googlesource.com/75150
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>