passing struct from Ada to OpenCL C

gistfile1.txt

// kernel

typedef struct {
   int2 offset;
   int2 center;
} viewport;

kernel void renderkernel(viewport vp) {
   // just accessing the struct crashes
   int a = vp.offset;
}

// calling code (Ada)
   type Viewport is record
      Center : Int2;
      Offset : Int2;
   end record;
   pragma Convention (C_Pass_By_Copy, Viewport);

   procedure Set_To_Kernel (Target : CL.Kernels.Kernel; Value : Viewport;
                            Argument_Index : CL.UInt) is
      procedure Setter is new CL.Kernels.Set_Kernel_Argument
         (Argument_Type => Viewport, Argument_Index => Argument_Index);
   begin
      Setter (Target, Value);
   end Set_To_Kernel;