Set Combo box value to Null

[TimTDP]

On a form I have two combo box's
The contents of the second combo box are filtered by the item selected in the first combo box.

If the value of the first combo box changes I want to delete the value in the second combo box

I have tried:
Forms![frmCustomer]![frmCustomerQuotation]![frmCustomerQuotationProduct].Form!cboProductId = Null

But get a run time error -2147352567 ~ You tried to assign the NULL value to a variable that is not a Variant data type. The data type is Numeric, Long

How do I do this?

Thanks in advance