oracle存储过程中带空值的输入怎么写?COALESCE()函数用错了吗?

oracle存储过程中带空值的输入怎么写?COALESCE()函数用错了吗?
(start_time in date,end_time in date,name in nvarchar2,tname in nvarchar2,cusers out sys_refcursor)
as

begin

open cusers for select * from C_USERS where logintime >= COALESCE(start_time,logintime)
and logintime