Parameter Query: using Like command does not return Null values when * is the parameter.

I use a form dialog box to select criteria for my parameter query that populates my main form.  On my dialog box the user can select * if they do not want to limit the values in the parameter query.  The issue I'm having is that using * as a parameter does not give me the null values.  I need it to give me the null values.  I've tried modifying the dialog box to say * or Is Null but that does not work from my dialog box but it works when I enter it directly in the query as such.  I need a parameter value that will give me all values including null if the user selects that criteria.  Here is the my parameter value in the query that populated from my dialog box: Like [Forms]![frmCRMFormGenerator]![cboWaitingOn].