Font disposing

When do I have to dispose TextureFont objects? I'm using it as a member of my own wrapper class and thought, I should dispose the existing object, before replacing them with a new one. But it appears, a fresh created TextureFont cannot get disposed?

OpenTK.Graphics.TextureFont testfont = new TextureFont(new Font("Arial",10.0f));
testfont.Dispose(); //throws NullReferenceException

I think, its because no texture has been attached to the font, right? But how can I tell from outside?

this is not the current svn, maybe this has already been fixed?