Dependent Combo Boxes and Null Values

[mrussell71]

I am running into a problem with null values within combo boxes which are dependent on other ones for their values. Each box is based on one underlying query. The combo boxes are cmbYear, cmbMake, cmbModel, cmbSubModel, cmbEngineType, and cmbLiter. cmbMake only shows the makes for the Year selected in cmbYear, and cmbModel only shows the models for the make and year already selected and so on.

The problem I have is the cmbSubModel box can contain a Null Value and when it is selected, the cmbEngineType and cmbLiter combo boxes end up being empty when there should be a resulting value. How can I allow Null Values to be used in the dependent combo boxes?

Thanks in advance.