A problem with "interface"

[Robert_Matthews <ignored@ramatthews.free-online.co.uk>]

In using interface types with GNAT I have encountered a problem.
Consider the following packages:

package Int is

   type A is interface;
   type B is interface;

   procedure P (Param1 : A; Param2 : B'Class) is abstract;

end Int;

with Int;
package Client is

   type Client_A is new Int.A with null record;
   type Client_B is new Int.B with null record;

   overriding
   procedure Client_P (Param1 : Client_A; Param2 : Client_B'Class);

end Client;

On compiling, GNAT says "Client_P" does not override anything.
Am I making a dumb mistake here - or is it GNAT?

The version of GNAT is GNAT GPL 2008 (20080521).

Thanks in advance,

Robert