请教这段代码到底什么有关问题?为什么会报错

Array
(
    [0] => Array
        (
            [id] => 3
            [fid] => 0
            [name] => 333
            [son] => Array
                (
                    [0] => Array
                        (
                            [id] => 2
                            [fid] => 3
                            [name] => 2_3
                            [son] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 4
                                            [fid] => 2
                                            [name] => 2_4
                                        )

                                )

                        )

                    [1] => Array
                        (
                            [id] => 5
                            [fid] => 3
                            [name] => 2_3
                        )

                )

        )

    [1] => Array
        (
            [id] => 1
            [fid] => 0
            [name] => 111
        )

)

function tree_add_level($tree){
    foreach($tree as &$val){
    	if(!isset($val['_level'])){
    	    $val['_level']=0;//顶层为0
    	}
		if(isset($val['son'])){
            $val['son']['_level']=$val['_level']+1; //下级加1
            tree_add_level($val['son']);
        }
    }
    return $tree;
}