This is basic validation and should be performed early

Fixes #12412

Change-Id: I903f7eeafdc22376704985a53d649698cf9d8ef4
Reviewed-on: https://go-review.googlesource.com/14110Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>