SICP 习题2.22 square迭代式写法的失败

代码产生的结果是((((()1)2)3)4)

而我们需要的是(1(2(3(4))))

(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons answer
(square (car things))))))
(iter items ()))

(define square
(lambda (x) (* x x)))

(square-list '(1 2 3 4))