Fixes #16910.

Change-Id: I0e97e88bee0361cd3621cacd1d4b84e6e42af718
Reviewed-on: https://go-review.googlesource.com/32424Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>