Cacading Update Combo box not refreshing

Hi i have used the following code to create cascade updating combo boxes. the code worked but as soon as i change the selection on the first combo the second combobox goes blank and does not refresh:

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1. Private Sub Combo18_AfterUpdate()
2.     With Me!Combo16
3.         If IsNull(Me!Combo18) Then
4.             .RowSource = ""
5.         Else
6.             .RowSource = "select chapter" & _
7.                          "From Tblchapters" & _
8.                          "Where Subject=" & Me!Combo18
9.         End If
10.         Call .Requery
11.     End With
12. End Sub

(Ps i am using access 2007)
Am i typing in somthing wrong or is extra code needed?