Fixes #15323.

Change-Id: I50e996e6fde6b24327cb45dd84da31deef4dcc56
Reviewed-on: https://go-review.googlesource.com/27171
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>