APEX

I have a form with several disabled items, including a select list with names of cities, which I have made disabled initially on page load. I enable it and the other items only when the user clicks on the button to enter new data. There is another select list on my page, which allows the user to select existing data. If the user selects something from there, I have a PL/SQL block to load the existing data into all my items without enabling them. I also have a validation to ensure that the value in the "cities" select list is not null. I fire that only when the form is being submitted. Now, the problem is that if the user selects existing data, my "city" value somehow gets erased from the session before the submit occurs, and my validation fails! I tried to use the "debug" link, and saw that before submit, it shows that the "city" value is assigned along with the other items, but on submit, the "city" value becomes null and validation fails. Anyone has any idea why this is happening? I am pretty sure it is something to do with the disabled select list! Everything works perfectly if the user clicks on "create new data" button. The main reason why I want the select list disabled is that if the user chooses existing data, I don't want him to modify the value in the "city" item, and then get an ugly database error!