Javascript exception popup even though code works fine

Question

I have made a fee submission form and using Javascript to do calculation of amount paid and amount due and change status according to result. Here is my code:

function ff_Student_Form_init()
{
  setInterval("AdmissionCalc()", 500)
}

function AdmissionCalc(){

ff_getElementByName('AdmissionFeeStatus').value='';
ff_getElementByName('AdmissionFeeBalance').value=Number(ff_getElementByName('AdmissionFee').value)-Number(ff_getElementByName('AdmissionFeePaid').value);

if(ff_getElementByName('AdmissionFee').value==0){
ff_getElementByName('AdmissionFeeStatus').value="Not Applicable";
}
if(ff_getElementByName('AdmissionFee').value>0){
if ((ff_getElementByName('AdmissionFee').value == ff_getElementByName('AdmissionFeeBalance').value)){

ff_getElementByName('AdmissionFeeStatus').value="Unpaid";
}
}
if(ff_getElementByName('AdmissionFee').value>0){
if ((ff_getElementByName('AdmissionFee').value != ff_getElementByName('AdmissionFeeBalance').value)){

ff_getElementByName('AdmissionFeeStatus').value="Instalments";
}
}
if(ff_getElementByName('AdmissionFee').value>0){
if ((ff_getElementByName('AdmissionFeeBalance').value == 0)){

ff_getElementByName('AdmissionFeeStatus').value="Paid";
}
}
 }

Unfortunately I am getting this exception popup but the script works fine can someone help.

*** EXCEPTION CAUGHT ***
PHP error level : E_NOTICE
PHP linenumber  : 7
Last known pos  : Before form custom piece code at line 1
Error message   : Undefined offset: 0

*** EXCEPTION CAUGHT ***
PHP error level : E_NOTICE
PHP linenumber  : 12
Last known pos  : Before form custom piece code at line 1
Error message   : Undefined offset: 0