Fixes #19592.

Change-Id: I8946b33fd36ae1f39bdcc4bf0bd4b5b99618efe8
Reviewed-on: https://go-review.googlesource.com/38300Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>