我的树形结构有什么问题？

Question

我试图弄清楚为什么树不打印。

 (define make-tree cons)
 (define root car)
 (define branches cdr) 

 (define t (make-tree 2 (make-tree 3 4)))

 (define (print tree)
     (if (null? (branches tree)) '())
        (sentence (root tree) 
                  (map (lambda(branch)(print branch))) (branches tree)))))

 (define (print2 tree)
     (if (null? (branches tree)) '())
        (sentence (root tree) (print2 (branches tree)))

 (print t)

我在stk-scheme中得到了错误的参数类型。