[Haskell-beginners] Error while using type and data constructor

Hi, I was writing some code for Tree. 1 : data Tree a = NULL | Node a (Tree a) (Tree a) works fine. But, 2 : data Tree a = NULL | Tree a (Node a) Tree a does not work. GHCI says Not in scope " type constructor or class 'Node'. Why is 2 declaration not valid though second definition looks much more closer to what a binary tree actually means ? Regards Nishant Verma