CB Code Field not Saving

Thank you very much for your effort and time to help...

I have created a "Code Drop Down (Single Select)" field. It is supposed to display a select input element showing the last 120 years. The user should select a single year. It displays the data correctly. However, it does NOT save the selected value. What am I missing?

Following is the code, which I entered in (Parameters -> Code):

$current_year = date('Y');
for ($i=0; $i<121; $i++)
{
$year = $current_year-$i;
$year_array[$year] = "$year";
}
return $year_array;