ActiveX Combobox in worksheet when populating the list based on another Combobox get error

When the selection chosen in Combobox3 is changed the following code runs. The issue occurs at the following lines

ComboBox1.List = .Range("AA2" & .Range("AA" & Rows.Count).End(xlUp).Row).Value

The actual error is Run-time error '381': Invalid property array index

any help identifying the issue would be greatly appreciated.

Looking at the property window for Combox1 the ListFillRange is empty, so it can't be this that is causing the error.

Private Sub ComboBox3_Change()

Dim indexDept As Long

indexDept = ComboBox3.ListIndex

ComboBox1.Clear

    Select Case indexDept
        Case Is = 0
            With Worksheets("Sheet2")
                ComboBox1.List = .Range("AA2" & .Range("AA" & Rows.Count).End(xlUp).Row).Value
            End With
    
        Case Is = 1
            With Worksheets("Sheet3")
                ComboBox1.List = .Range("AA2" & .Range("AA" & Rows.Count).End(xlUp).Row).Value
            End With
  
    End Select
 
End Sub