Ajax Call To [Object%20Object]

I have a link that fires an AJAX request inside a JavaScript function that will render a pop-up form. This pop-up form is actually an 'edit' form, meaning there is data already filled in each form element.

This is the code for the link:

CHtml::link('Update', "",  // the link to open the dialog box
array(
'style'=>'cursor: pointer; text-decoration: underline;',
'onclick'=>"{updateAgent(" . $agent['id'] . "); $('#dialogUpdateAgent').dialog('open');return false;}"));

This is the updateAgent() JavaScript function:

function updateAgent(id)
{ 	
	var update_id;
	update_id = '?r=site/updateuser&id='+id;
	
    <?php
	echo CHtml::ajax(array(
	'url'=>'js:update_id',
	'data'=> "js:$(this).serialize()",
	'type'=>'post',
	'dataType'=>'json',
	'success'=>"function(data)
	{
		if (data.status == 'failure')
		{
			$('#dialogUpdateAgent div.divForForm2').html(data.div);
			$('#dialogUpdateAgent div.divForForm2 form').submit(updateAgent);
		}
		else
		{
			$('#dialogUpdateAgent div.divForForm2').html(data.div);
		}		
	}",
	));
	?>
    return false; 
}

Problem:
The form does not seem to submit. And when I check the console in firebug, it causes "NetworkError: 500 Internal Server Error" everytime I hit the submit button of the form with this url: "site/updateuser&id=[object%20Object]".

The form will only submit successfully if I will hard code the id value of the JS function like this:

update_id = '?r=site/updateuser&id='+3;

instead of

update_id = '?r=site/updateuser&id='+id;

I researched about issues regarding the passing of JS variable to Ajax and did some tests but still the problem persists.

Thanks in advance!