Without this, T can sneak through to the backend
with its width unknown.

Fixes #20174

Change-Id: I9b21e0e2641f75e360cc5e45dcb4eefe8255b675
Reviewed-on: https://go-review.googlesource.com/42175
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>