json_decode() expects parameter 1 to be string

public function actionUpdateVoteOnce(){

        $postdata = '[
        {"mobile": "145682796533",
        "answers" : [{"id" : "104", "is_mcq" : "1", "option" : "voteCount4", "freeText" : ""},
        {"id" : "105", "is_mcq" : "1", "option" : "voteCount4", "freeText" : ""},
        {"id" : "106", "is_mcq" : "1", "option" : "voteCount4", "freeText" : ""},
        {"id" : "109", "is_mcq" : "0", "option" : "", "freeText" : "Test loop 2"}]
        }]';

         $records = json_decode($postdata);

        $mobile = $records[0]->mobile;


        foreach ($records[0]->answers as $rec) {


            $is_mcq = $rec->{'is_mcq'};

            if ($is_mcq == 1){
                $id = $rec->{'id'};
                $ans = $rec->{'option'};

                $voteData = array();

                $record = Vote::model()->findByPk($id);

                if ($record) {
                    $record->total = ($record->total) + 1;
                    $record->$ans = ($record->$ans) + 1;
                    $record->save();
                }

                array_push($voteData, $record);


                $outObj = array('success' => true, 'data' => $voteData, 'info' => 'Ok');
                echo CJSON::encode($outObj);


            } elseif($is_mcq == 0){

                $freeAns = $rec->{'freeText'};
                $id = $rec->{'id'};

                $rec = new VoteFreetoType;
                $rec->q_id = $id;
                $rec->answer = $freeAns;
                $rec->mobile = $mobile;
                $rec->date = date("Y-m-d H:i:s");
                $rec->save();


                $outObj = array('success' => true, 'info' => 'Ok');
                echo CJSON::encode($outObj);
            }

        }

}

$postdata= json_decode($_REQUEST);