Fixes #15061.

Change-Id: I71f69f398d1c5f3a884bbd044786f1a5600d0fae
Reviewed-on: https://go-review.googlesource.com/21398
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>