parameter problem

Hi,

I created a report base on a data range parameter, and in the report
property event:
On Open: Date Dialog.Open Dialog
On Close: Date Dialog.Close Dialog

But everytime after I entered the beginning date and Ending date:
There is always showing a message:
The expression you entered has a function name that MS can't find.

I ignor it, click on OK, the report still open.

I can't figure out which function name is wrong.

If I delete the event setting in the property, then has no problem.
But I do need the Date dialog for the people to enter.

Thank you for your idea.