How can I use an optional array argument in a VBA procedure?

I've got a private procedure in a VBA script for MS Access:

Private Sub drawLineDiagram(chartSpace As Variant, title As String, caption As String, x_val() As Variant, y_val() As Variant, Optional y_val2() As Variant = ????)

As you see, I want to have an optional last parameter for an array of values.

What kind of default parameter must I assign? If I do it with an optional integer value and assign it e.g. 0 it's all fine.

If I do it with an array as shown above and assign an array, the line is marked red => as an error (and it won't compile).