Populating A Text Field From A Combo Box Selection.

I have a combo where the list is populated with a bit of VB code to create a two field list. When the user selects the item they want from the list it displays the item in the combo box and should populate a text box in the same row with the value from the second column.

This combo is part of an orders subform and the user is selecting a product from the list which displays the product_name and the product_id.

The problem is that the product_id is not filled in and access returns the following error:
Run-time error ‘3101’:
The Microsoft Jet database cannot find a record in the table ‘product_table’ with key matching field(s) product_id.

This is the VB code that populates the list :
[code]
Private Sub comboCatagory_ID_AfterUpdate()
Dim sProd_description As String
sProd_description = "SELECT products_table.product_id, products_table.prod_description " & _
"FROM products_table " & _
"WHERE products_table.prod_catagoryID = " & "'" & Me.comboCatagory_ID.Column(0)
Me.comboProd_description.RowSource = sProd_description
Me.comboProd_description.Requery
End Sub
[end code]

What am I doing wrong?

Thx
Kev.