debugger error

(defclass standard-point ()
        ((x :initform 0 :initarg :x)
            (y :initform 0 :initarg :y)))


(defgeneric distance-origin (point))

(defmethod distance-origin ((point1 standard-point))
        (when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y))
                (sqrt (+ (expt (sb-mop:standard-instance-access point1 0) 2)
                                                    (expt (sb-mop:standard-instance-access point1 1) 2)))))

(defgeneric distance (point1 &optional point2))

(defmethod distance (point1 &optional point2))

(defmethod distance ((point1 standard-point) &optional point2)
        (when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y))
                (sqrt (+ (expt (sb-mop:standard-instance-access point1 0) 2)
                                                    (expt (sb-mop:standard-instance-access point1 1) 2)))))

(defmethod distance ((point1 standard-point) &optional (point2 standard-point))
        (when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y)
                                                    (slot-exists-p point2 'x) (slot-exists-p point2 'y))
  (sqrt (+ (expt (- (sb-mop:standard-instance-access point2 0) (sb-mop:standard-instance-access point1 0)) 2)
                                            (expt (- (sb-mop:standard-instance-access point2 1) (sb-mop:standard-instance-access point1 1)) 2)))))

(defvar p1 (make-instance 'standard-point :x -3 :y 5))
(defvar p2 (make-instance 'standard-point :x 7 :y -1))

(defun test ()
        (distance p2 p1))

(test)