Extending TImage

Hello,
could someone please tell me why TImage always return Null, if i try to cast it:

Type TMyImage Extends TImage

        'result = Null
	Function Open:TMyImage (url:Object)
		Local image:TMyImage = TMyImage(TImage.Load(url,MASKEDIMAGE,255,0,255))
		Return image
	End Function

        'result = null
        Function Open2:TMyImage (url:Object)
                Local image:TImage = TImage.Load(url,MASKEDIMAGE,255,0,255)
                If image Return TMyImage(image)
         End Function

End Type

I've tested several ways, but casting TImage doesn't seem to work.