Is this a bug?

[freeagle]

I have a class BaseWindow
{
Dimension2D size()
	{
		return _size;
	}

abstract void size(Dimension2D size);
}

and derived class Win32Window : BaseWindow
{
void size(Dimension2D size)
{
	_size.set(size);
}
}

lets say i have a reference to class Win32Window. When i try to call 
method _window.size(), compiler complains it doesn't match arguments 
size(Dimension2D). Until i specify is to 
(cast(BaseWindow)_window).size(). Why??