title: Racket Error: Expected a procedure that can be applied to arguments
body: <p>Sorry, I just started using racket. I'm pretty new at this. I've written this piece of code:</p>
  
  <pre><code>(define (save_sheets lst num)
    (if(= num (- (length lst) 1))
       ('())
       (
        (my_save_image (join_sheet (list-ref lst num)) num)
        (save_sheets lst (+ num 1)))))
  </code></pre>
  
  <p>Of course, when I run this I get this error:</p>
  
  <pre><code>application: not a procedure;
  expected a procedure that can be applied to arguments
  given: (object:image% ...)
  arguments...: [none]
  </code></pre>
  
  <p><code>(join_sheet (list-ref lst num))</code> should return an image, which the error shows, but the <code>my_save_image</code> should take it in right? It's parameters is the image and a number. Thanks!</p>