where keyword and COmparable

Hello,
 I read this code online and it doesn't compile; i wonder where the error and hopefully a simple link to cope this topic
    class BinaryTree <T> where T:IComparable
    {
        private T dataValue;
        private BinaryTree leftSubTree; // here error
        private BinaryTree rightSubTree; //here error
...............................................
}

Regards,