Fixes #20839.

Change-Id: I125460c5da09b7fa0cf470ff5be436f8d650cde7
Reviewed-on: https://go-review.googlesource.com/47253
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>