NaN on the run (add cell contents) [CS3, JS]

Question

Hello.
This is my first attempt at javascripting ID, though I've done some amount of Applescripting.
The intent of this script is to add the contents of the table cells that a user has selected.
I can't figure out why this keeps returning 'NaN' (not a number) after I have told it to ignore any 'NaN' contents (and exclude them from the running total).
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
var theTotal=0;
//If the selection is at least one table cell...
if (app.selection[0].constructor.name == "Cell")
//count cells in selection
var cellCount=(app.selection[0].cells.count(0));
//repeat with count from 1 to count of cells
for (a=0; a<=cellCount; a++)
//get contents of first selected cell in the loop
cellVal=(app.selection[0].contents[a]);
//convert string to # via parseFloat command
cellValInteger=parseFloat(cellVal);
if (cellValInteger!="NaN") //NaN = not a number
//set the variable to the sum of each cell value
theTotal+=cellValInteger;}
else{
alert("Please fully select at least one table cell and try again.");

Answer 1

Bob and Dave, many thanks for your help. It works now, and my next challenge is to figure out how to put that data into the last cell.
Aaron