NaN why?

1

Can anyone explain to me why I can not get the numeric values for lon_zoom and lat_zoom???
everthing else is returning the correct numbers???

var This_Lat:Number = address.attributes.lat;
  var This_Lon:Number = address.attributes.lon;
  
var lon_zoom:Number = (global_lon+This_Lon)/2;
var lat_zoom:Number = (global_lat+This_Lat)/2;

var latlonZm = new LatLon(lat_zoom,lon_zoom);

trace(lat_zoom+"="+global_lat+"+"+This_Lat);

The trace returns this

NaN=51.48408+51.32121