Concantenating values selected from comboboxes

Hi all,
Having a bit of a problem trying to get this to work: I have a userform with several comboboxes. When I select a value from combo_material a second combo box appears (combo_material_type). When I make a selection from this combobox I would like that value to be added (concatenated) to the value from combo_material. Here is a snippet of code:

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When I run it, the second combo box appears, however when I select "CRS" it is not added to that of the value selected in combo_material

Thanks in advance!
Chris

Bump, maybe I mistitled this...............