-1067: Implicit coercion of a value of type spark.components:TextInput to an unrelated type Number

I am new to flex i was tring to get the converted value of celsius from  in a text input i wrote the folowing code :

override public function onRegister():void

                    {

                              eventMap.mapListener(view.btnCelsius,MouseEvent.CLICK,onClickCelsius);

                    }

                    public function onClickCelsius(vEvent:MouseEvent):void

                    {

                              view.txtCelsius =  String((view.txtFarhenheit-32)/1.8);

                    }

got the 1067 error please correct me.