php to pdf checkbox not working with my code

what i am doing wrong,please guide me.

My pdf fields '''FieldType: Button FieldName: aaaa FieldFlags: 0 FieldValue: Off FieldJustification: Left FieldStateOption: Off FieldStateOption: Yes'''

here is my code

require('fpdm.php');

$fields = array(
'POPMLSAddress' => $_POST['Text3'],

'aaaa'=> 'Yes'
);

$pdf = new FPDM('check1.pdf');
$pdf->Load($fields, false); // second parameter: false if field values 
are in 
ISO-8859-1, true if UTF-8
$pdf->Merge();
$pdf->Output();

ob_end_flush(); 
}</code></pre>

in pdf file checkbox have to be checked with tick