form_dropdown() always returns 0

[eluser]EthanSP[/eluser]
Why is it that this form_dropdown() always returns 0?

form_dropdown($fby,$fy_values,$fy_default) // in view

where,

$fby is $data['fby'] = array('name'=>'birth_year'); // in model

$fy_values is $data['fy_values'] = $datelib->show_years(); // in model

function show_years() {
for ($x=date('Y')-150; $x<=date('Y'); $x++)
$years[] = $x;
return $years;
}

$fy_default is $data['fy_default'] = array_search(date('Y'),$data['fy_values']); // in model

In the controller:

if($this->input->post('mysubmit')) {
$this->mdl_demographics->save();
}

In the model:

$db_array = array('last_name' => $this->input->post('birth_year'));

Result in db:

last_name = 0

Why?