Forms :: List-item Selection Not Working?

i am facing a problem in functionality of a form as there are different search parameters:
date from: text item-->mandatory
date to: text item---->mandatory
house type: list item-->optional
form_status: list item->optional
order by: list item---->optinal

untill i don't select house type by list item everything is working fine and gives desired result, even with house type if i select any house type result is fine but as i select last list item of house type which is null as i want result without house type then it gives no any data as data is present in database.

for example:
date from: 20-05-2009
date to: 21-05-2009

with these two i got the correct result, then i select
house type: 3

i got the correct result and when i change list item to any value like: 1,2,3.....it is working fine but as i want result without house type by selecting the null value in list item as:
date from: 20-05-2009
date to: 21-05-2009

it is not picking any data from database. but the same search criteria in 1st case as i described above giving result.
declare
qry varchar2(5000);
n number;
alert number;

[code]...