[plt-scheme] How to change the following code to make it handle nested lists?

I have following code:

(define (mysum list)
   (if (null? list)
       0
      (+ (car list) (mysum (cdr list)))
    )
)

It can handle the case summing up a list of numbers....however, I want
to handle case like '(1 (2 3) 4), how can I make it?
The code can handle (mysum '(1 2 3)) => 6, but when it comes to (mysum
'(1 (2 3) 4))=>error.  Any help would be appreciated.