autocomplete javascript error

hi to everyone,
I made a form page, but I'm not usign drupal_get_form function to do that, but I made form by myself.
But, I would like to use autocomplete textbox in some fields and I make this function:

function _pratiche_istituti_concorrenti_form($idIstituto)
{
	$form = array(
		'#name' => 'istituto_impegno[]',
		'#type' => 'textfield',
		'#autocomplete_path' => 'myauto/autocomplete',
		'#value' => $idIstituto,
  		'#size' => 15
	);

	return drupal_render($form);
}

Textfield is rendered very well, but Firebug gave me this error:
input is undefined
[Break on this error] $(input.form).submit(Drupal.autocompleteSubmit);\n

is there any way to fix that error? Thank you very much