oo problem help please

[shoko2004@hotmail.com (shoko)]

what is wrong with the following code:

----car.ads----
package car is
  --car class --
  type car is tagged private; 
  type car_ptr is access all car'class;
  
  --car methods --
  procedure set_name(name:String;this:in out car);
  function  get_name(this:car) return string;
  
 private
     type car is tagged 
     record
         name:String(1..256);
     end record;
  
   
end car;
---car.adb---
package body car is
  procedure set_name(name:String;This:in out car)is
  begin 
     this.name:=name;
  end set_name;
  
  function  get_name(this:car) return string is
  begin 
     return this.name; 
  end get_name;  
end car;
----transport.ads-----
with car;

package transport is
   type transport_rec is new car.car with private;
   type transport_Ptr is access all transport_rec'Class;

   function init_transport(maximum_speed:positive) return transport_Ptr;
   
   private
     type transport_rec is new  car.car with null record;
         
end transport;

-----transport.adb----
package body transport is
   function init_transport(maximum_speed:positive)return transport_Ptr is
        This: transport_Ptr;
   begin
      This:= new transport_Rec;
         
      return this;
   end init_transport;
  

end transport;

----------bus.ads---
with transport;

package bus is
   type bus is new transport.transport_rec with private ;
   type bus_Ptr is access all bus'Class;
   function initbus(name:string) return bus_Ptr;
   private
      type bus is new transport.transport_rec with null record;
end bus;    

---------bus.adb--------
package body bus is
   function initbus(name:String) return bus_Ptr is
       This: bus_ptr;
   begin
      This := new bus;
      set_name(name,this.all);   <--  CONSTRAINT ERROR
      
      return this;
   end initbus;

end bus;
------------------------------
and finally

--------main.adb--------------

with car;use car;
with bus;use bus;


procedure main is
   cars:array(1..2) of car_ptr;
   new_bus:bus_ptr;
   
  
begin
 
     cars(1) := car_ptr(initbus("mercedes"));
  
end main;


the problem is that i get constraint error when the code is executed
why???