Fixes #13365.

Change-Id: I5a447ff806dbbb11c8c75e2b5cfa7fd4a845fb92
Reviewed-on: https://go-review.googlesource.com/17206
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>