notice:use of undefined constant: add-assumed 'add' in..........

i have something like this: if(!empty($_SESSION['add']['start_date']) && !empty($_SESSION['add']['end_date'])..........)){ and i get the unexpected error: notice:use of undefined constant: add-assumed 'add' in (filename)..... dont quite understand why that is since i used other code that i have that works just like that (just different var names) and since that works i thought it would work for this one too??? but it doesnt?? im really lost now since i defined what $_SESSION['add']['...'] was already in the file before it...just like the rest of the stuff..... any ideas whats wrong??