Using A Combo Box Selection For Query Criteria

I have a report that is based on a query. In the query, I have a field called "Mode". In the criteria section for the Mode field, I am calling the selection from a combo box on a form called "frm_main". So the criteria for the Mode field is "[Forms]![frm_Main]![Mode_ComboBox]". This combo box has selections for 1,2,3,4,5. I want a selecton on the combo box that will work with the query to show all modes. Sometimes something weird gets entered into the database like "NoMode" for example. I have tried adding a combo box selection of "*" , "Is Not Null" and "". If I manually type these into the criteria section of the query, it works fine. But when I use these in the combo box and call the combo box selection from the query, it does not work.

Does anyone have any idea what I can use in my combo box selection that when selected, the query (and hense report) will show all data?

Thanks,
Jim