button that copies all data in repeating subform fields to 2nd subform

I am trying to make my form so that a button will copy all entries that are in repeating subform A into the same fields in repeating subform B. I have a possible script but it throws an error message when there is not another line in subform A. For example if I have 2 sets of the subform A, the button will say that P1.invoice[2] (the third set) cannot be found. The script does copy and paste the data into the correct fields in subform B.
Here is a shortened version of my current script:

//if the [0] row exists in the first dynamic itemset, then copy the data from it into the second dynamic item fields.

var copy0i = form1.P1.ItemSet[0].Invoice.rawValue;
var copy0a = form1.P1.ItemSet[0].Amount.rawValue;
if(Exists(form1.P1.ItemSet[0].Invoice))
then
form1.ItemSetPage.Return.ItemSet[0].Invoice.rawValue = copy0i
form1.ItemSetPage.Return.ItemSet[0].Amount.rawValue = copy0a
endif

var copy1i = form1.P1.ItemSet[1].Invoice.rawValue;
var copy1a = form1.P1.ItemSet[1].Amount.rawValue;
if(Exists(form1.P1.ItemSet[1].Invoice))
then
form1.ItemSetPage.Return.ItemSet[1].Invoice.rawValue = copy1i
form1.ItemSetPage.Return.ItemSet[1].Amount.rawValue = copy1a
endif

var copy2i = form1.P1.ItemSet[2].Invoice.rawValue;
var copy2a = form1.P1.ItemSet[2].Amount.rawValue;
if(Exists(form1.P1.ItemSet[2].Invoice))
then
form1.ItemSetPage.Return.ItemSet[2].Invoice.rawValue = copy2i
form1.ItemSetPage.Return.ItemSet[2].Amount.rawValue = copy2a
endif

If anyone can either (1) correct this script so that it works but without the error message or (2) suggest a different approach to scripting it, I would really appreciate it.