Common LISP - Should be a lambda expression

Kaere eksperter, jeg forstaar hvad "Should be a lambda expression" betyder. Her er output, koden er nedenunder.

====  error msg ====

[16]> (load "missionaries-and-cannibals.lisp")
;; Loading file U:\ai\ass\missionaries-and-cannibals.lisp ...
*** - SYSTEM::%EXPAND-FORM:
      ((EQUAL '(BOAT-SIDE STATE) (PEOPLE-COMING-FROM-SIDE))
        (- NUMBER-OF-PEOPLE-ON-SIDE NUMBER-TO-BE-MOVED))
      should be a lambda expression
The following restarts are available:
SKIP          :R1      skip (DEFUN UPDATE-PEOPLE-AMOUNT # ...)
STOP          :R2      stop loading file U:\ai\ass\missionaries-and-cannibals.l
isp
ABORT          :R3      ABORT
Break 1 [17]> abort
[18]>



==== program kode ====

(defun update-people-amount (number-of-people-on-side number-to-be-moved people-coming-from-side)
    cond(
        ;; if boat and people are from same side then subtract number-to-be-moved from number-of-people-on-side
        (
            (equal (boat-side state) (people-coming-from-side))            ; if people and boat are on same side
            (- number-of-people-on-side number-to-be-moved)                    ; return the updated number of people
        )

        ;; if boat and people are from opposite sides then add number-to-be-moved fromto number-of-people-on-side
        (t (+ (number-of-people-on-side) (number-to-be-moved)))            ; return the updated number of people
    )
)

Nogen forslag?