Project Euler Problem 12 Common Lisp

Project Euler Problem 12

For function problem12, every time it loops, it calculates the next triangle number (i).  It keeps looping until it reaches a triangle number that has over 500 factors.

(defun how-many-factors? (num)
  (* 2 (loop for i from 1 to (isqrt num)
             if (zerop (mod num i))
             count i)))

(defun problem12 ()
  (loop for i = 1 then (+ i j)
        and j upfrom 2
        when (> (how-many-factors? i) 500)
        return i))