Returned Value in Subject Line is 1 less than Entered

Hello,
 
I am fairly new to Acrobat, but this is the first problem I have run into I haven't been able to find a solution to.
 
I am creating a document to submit warranty information based on VIN number. I am inserting the VIN number into the subject line and it always ends up 1 less than the user entered.
 
EX:
user enters: 12345678901234567
returned value: 12345678901234566
 
I have tried just tacking a +1 into the code and it just returns a number 1 too big and returns an value with an 8 on the end.
 
here is my code:
 
var VIN = this.getField('VIN').value;
var cToAddr = "(EmailAddressHere)"; //removed email address
var cSubLine = "Warranty Labor Request Form for VIN#: " + VIN;
var cBody = "Thank you for submitting your form.\n" + "Save the mail attachment for your own records";
 
this.mailForm({bUI: true, cTo: cToAddr, cSubject: cSubLine, cMsg: cBody});
 
I was thinking about just using a string so that the exact value is transfered over, but curiosity is getting the better of me here and I would like to know why it is returning the wrong value.
 
Thank you so much for any help.
 
Joe