Creating Report Displaying Leads without any Open/Closed Activities

Hi All,  

I'm currently trying to build a report which shows the number of Leads which do not have any activities assigned to them. Thus, I've built a basic view, using First Name, Last Name, Lead ID and # of completed activities.   To filter the report (I selected an "activities" report) to ID only those who do not have any open or completed tasks assigned to them, I've tried filtering the "# of Activities" using both the "is null" and "equal to 0" filters (not at the same time, but seperately). When I go to preview the report, it says that there