Jquery Ajax callback

I've got an ajax call via jquery that executes without error until I get to the callback. The content returned looks like this:

{"UPSELLABLE":true,"OFFERTEXT":"p47r.cfm"}

Simply doing alert(upselldata); will alert the data above. But if I try access variable upselldata like a javascript object( I thought jquery did the eval work for me already ), the variables are undefined. See code below:

        $.ajax({
         type: "POST",
         datatype: "json",
         data: "ProductID=1",
         url: '/templates/public/upsell_available.cfm',
         success: function(upselldata) {               
          alert(upselldata.UPSELLABLE); // upselldata.upsellable is undefined!?!?!
         }
        });