General :: Yes / No Displaying As -1 / 0 In Second Form

I have a table which has a yes/no field, in the table everything displays as either Yes or No, unless you click on it to edit it, whereby No become 0 and Yes becomes -1.This isn't a big problem as users can only access forms. On the input form this is a check box, checked for Yes and unchecked for No. This used to be a drop down selection box of Yes and No and the same problem was occuring, so I know the problem doesn't relate to the check box.

I have a second reporting form which has multiple combo boxes to filter down results in a subform. In the subform the field correctly displays as Yes and No, however, the filtering combobox for this field has the options 0 and -1.

The combobox is set up as Yes/No, the table is Yes/No and the results are Yes/No. There is no other formatting or input options that can be changed to tell the database this is a Yes/No field.How can I get this combobox to display as Yes/No? It's causing a lot of confusion with users as they are having to select 0 or -1.As a note, I've already tried the following:

-Deleting the combobox and starting again.
-Deleting the table and combo box and starting again.
-Deleting all tables, queries and forms which use this field and starting again.

None have worked, this seems to be something Access is doing rather than an error in what I've done.The filter form uses this code, which may be the cause:

Code:
Option Compare Database
Option Explicit
Private m_colCombos As Collection
Private m_strFilter As String

[code]....