Optional in @prompt function

Hi,

I have created an optional  filter using @prompt function @prompt('Parameter Name','A','LOV',Mono,constrained,not_persistent,,User:1,optional). But it show below error while refreshing the report.

Database error: ORA-00936: missing Expression.(IES 10901) (WIS 10901).

can we provide "optional" in @prompt function.

Thanks,

Sathish