Adding dynamic text box values - NaN

Hi all,
I have a movie that contains a series of radio button groups, which each have their own accompanying dyn text box.
When a certain radio button is selected, a pre-determined value is loaded into each respective groups’ text box, via “onRelease”.

Now, I can get these values to appear fine in each text box, but I then have a fourth text box (amount_total), into which I want to have the sum of all the previous text box’s values.

I have a submit button with the following actions on the first frame of the timeline:

onSubmit = function (){
_root.amount_total = Number(_root.total_surf) + Number(_root.total_music) + Number(_root.total_video);

Now, when I hit the submit button NaN is returned.
Any ideas as to what the possible issue is here ?
FLA posted