bagaimana mengatasi error array to string di dropdown group multiselect codeigniter?

Bagaimana mengatasi error array to string di dropdown group multipleselect codeigniter? berikut saya lampirkan code nya :

model

 function getZonaTree2() {
		$types = $this->getZonaType();
		$tree = array(''=>'-- Region --');
		$this->getListTreeParent($listZona, 0, 0);
		foreach($listZona as $r) {
			if($r->LEVEL==1) {
				$tree[$r->ZONA_NAME] = array();
				$zona_parent_1 = $r->ZONA_NAME;
			}
			if($r->LEVEL==2) {
				$tree[$zona_parent_1][$r->ZONA_NAME] = array();
				$zona_parent_2 = $r->ZONA_NAME;
			}
			if($r->LEVEL==3) {
				$tree[$zona_parent_1][$zona_parent_2][$r->ZONA_ID] = $r->ZONA_NAME." (".$r->ZONA_CAPITAL.") - ".$types[$r->TYPE];
			}
		}
		return $tree;
	}

view

 <?php
        echo form_multiselect('zona_id', $this->Zona_model->getZonaTree2(), array(), "size=8 id='zona_id'");
      ?>

tampilan errornya :

tampilan seharusnya :