Removing the 'unassigned' group from a list grouped view

I have a list that I'm using to capture training information for employees.  There is a choice of 'cards' or 'certificates' in each record and I have set up a view to show each.  In the 'card' view it's showing the 'certificates' records as unassigned (because they are not related to cards, only certificates).  I've tried to filter this view such that the 'unassigned' is not visible, however I can't get it to work.  I've tried to Filter the view as per the attached screen shot and also using null as a value but none of that has worked.  Can anyone help please.