MS Access - Changing Query Criteria based on checkbox

I've got a Query running which works fine now. It lists all transaction done. I want to add a checkbox to the Form that uses the Query to filter out any transaction with a Cancelled Date listed on it. I've tried adding a Criteria to the CancelDate column on the query like this:

=IIf( Forms![Show DPA List]![chkShowCanc] =true,"*","IsNull")

And I keep getting a 'This Expression is typed incorrectly' error....

Is there a better way to approach this?