what's wrong with json_encode with mysqli, help meee..

[eluser]elevend[/eluser]
i have problem when i send echo json_encode(data) from server and cannot receive in client.
i'm using mysqli for multiple batch query.

in server, i had insert using stored procedure after that i send json encode,
like this code in bottom:
code in my server

Code:

function doInsert(){
$queryanswer = "CALL proc_insert_tbmsanswer(?,?,?,?,?)";

$resultanswer =  $this->All_model->query_data($queryanswer,$dataanswer);

echo json_encode($resultanswer);
}

code in my client

Code:

$.post("<?php echo base_url()?>Support/doInsert",
                      {
                            typeInsert: "answer",
                            createdAnswerByQuestion : ("#questionId").val(),
                            descriptionAnswer : ("#answerfield").val(),
                            createdByAnswer:"<?php echo $username; ?>",
                            createdIdAnswer:"<?php echo $userid; ?>"
                      },function(data){
                            alert(data);
                      },"json"
                );

in my client code,, i had alert(data) or alert something,
but it not to alert..
what's wrong it's that?

note: if i had using mysql it's work, but mysqli it's not work.

please help me and thank you..