执行触发器时触发错误

Question

在创建触发器iam时出现错误： 子查询返回的值超过1。当子查询遵循=，！=，<，<=，>，> =或将子查询用作表达式时，不允许这样做。

UPDATE rm 
        SET 
            rm.cost_sqft = isnull((
                select case when (rm.rm_cat in ('8','NON-REPORT') and rm.space_fee = 'No') then '0' else rent_w.w*@rental_rate end 
                from @rental_weight as rent_w, bl, rm            
                where rm.main_contact=rent_w.space_type
                and rm.description=rent_w.space_quality
                and bl.cost_type = rent_w.campuse_cost      
                and bl.bl_id = rm.bl_id
            ),'0') 
                WHERE rm.bl_id=@bl_id
        AND rm.fl_id=@fl_id
        AND rm.rm_id=@rm_id 

        FETCH NEXT FROM rm_cost
        INTO @bl_id, @fl_id, @rm_id         
    END