You might use CocoaDialog instead:
set l to {"a a", "b"}
set l2 to ""
repeat with i in l
set l2 to l2 & quoted form of i & " "
end repeat
do shell script "/Applications/CocoaDialog.app/Contents/MacOS/CocoaDialog \\
standard-dropdown --title Title --text Text --items " & l2
set {button, answer} to paragraphs of result
if button is 1 then return
item {answer + 1} of l