macOS AppleScript Studio List Item Help

[AchillesII]

After working on this problem for over an hour, I still can't figure out how to get the number of items in a list - and display that number in a text field. For some reason this code doesn't work (though it should) and is the same code that I normally use for regular AppleScript. This code when run displays a "the variable stringNum is not defined" error though the var stringNum is clearly defined below.

Code:

property theString : {"a", "b"}
on clicked theObject
set stringNum to number of items of theString
set contents of text field "textField" to stringNum
end clicked

Thanks in advance...