command-execute in .emacs file

[fab]

Hi all!
I'm a beginner with lisp language and I'm trying to use Emacs with Speech
Dispatcher 
and Festival (in English language).

This manual page [1] says how to remove the brltty symbol from the
list of active output drivers, using the interactive command:

> M-x speechd-out-disable-driver RET brltty RET

Now I'm tryng to customize my .emacs file, but if I use this form

> (command-execute 'speechd-out-disable-driver nil "brltty")

I obtain this error message:

> Wrong type argument: vectorp, brltty

Can anyone help me? Thanks!

Fabio

[1] http://cvs.freebsoft.org/doc/speechd-el/speechd-el_9.html#SEC16
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