How to restrict Null user response

Hi,

I have a parameter field with type as 'String'. I am converting it to number due to user requirement.

{PR.ID} = tonumber({?PR ID}).

problem is even if user dont enter anything, the report is getting generated as blank since its taking user response as ("") Null.

If i have the type as 'Numeric' it will display a warning as 'Please enter a value'.

So user wants the same warning to be displayed when a NULL value is entered. Is that possible?

The report has only one prompt and it is not optional.

Regards,

Siva.