Parameters with radiobutton

Hi,

I want to use only one radiobutton on the selection screen.

I have declared the parameters statement like this.

PARAMETERS: p_screen RADIOBUTTON GROUP rad1.

When I try to execute the program it is giving error as

'The radiobutton group "RAD1" contains only one PARAMETER'.Can anyone tell me what would be the reason for this.

Regards,

Hema