未定义偏移量:0错误

一切正常，但页面显示错误:-

定义抵消:0 .

for($i=0;$i<count($darr12);$i++){   
    $pro12=$this->Process->find('all',array('fields'=>array('Process.id','Process.subgroup','Process.frequency','Process.USL','Process.LSL','Process.UCL','Process.LCL','Process.range'),'conditions'=>array('Process.part_no'=>$darr12[$i]['samples']['part_no'],'Process.name'=>$darr12[$i]['samples']['process'],'Process.characteristic'=>$darr12[$i]['samples']['characteristic'])));
    $us12=$pro12[0]['Process']['USL'];//error
    $ls12=$pro12[0]['Process']['LSL'];//error
    $rg12=$pro12[0]['Process']['range'];//error
}