Submit button in Autohotkey GUI based on dropdown

Question

Via autohotkey I'm trying to

1. Read the contents from a dropdown box
2. Present the content to the user in a AHK GUI dropdown box
3. Select the correct item in the original dropdown box via the input from the AHK dropdown box.

I have the following code so far:

CClass  = fpOCXComboBox209
Title   = ahk_class ThunderRT6FormDC

ControlGet, ItemList, List,, % CClass, % Title
Gui, Add, DropDownList, vDDL, % RegExReplace(ItemList,"`n","|")
Gui, Add, Button, gSetChoice, Select ; gLabel jumps to action.
Gui, Show, w300
return

SetChoice:
    GuiControlGet, CurItem,, DDL
    Control, ChooseString, % CurItem, % CClass, % Title
return

GuiClose: ExitApp

The behavior when I test it, is when i press "Select" in the Gui dropdown box, the original dropdownbox is blanked (as if AHK tries to add a blank variable or something like that)

I believe the error might be in the last line "Control, ChooseString, % CurItem, % CClass, % Title", but I can't see why.

Anyone able to help me out?