Emultiselect Help me Please

mb_strlen() expects parameter 1 to be string, array given

ive got this problem and i dont how to fix it…im just new to this framework…can someone help me regarding this issue…

this is my code on view

<div class="row">



        $dbval = Faculty::model()->findAllbySql('SELECT id, CONCAT(Firstname, " ",Middlename," ", Lastname) as Lastname FROM tblfaculty ');

	$list = CHtml::listData($dbval, 'Lastname','Lastname');


	    echo $form->dropDownList(new Setpeer,'setpeers',$list,array('multiple'=>'multiple','key'=>'setpeers', 'class'=>'multiselect'));



   <?php echo $form->error($model,'setpeers'); ?>


thanks in advance