Type Error got NIL, was expecting NIL

[Grant Haywood]

Hello,
I'm new to extempore, and scheme in general, so forgive me if there is an obvious mistake here, however I have the following situation:

I have defined sum in plain scheme as the following, and it works fine in Extempore:

(define ssum

  (lambda (lst) 

    ( cond

        ((eqv? (cdr lst) ()) (car lst))

        (#t 

         (ssum (cons

                        (+ (car lst) (car (cdr lst)))

                        (cdr (cdr lst))

                )

              )

        )

    )

))

for example

(print (ssum (list 1 2 3 4)))

results in 10

however when i define the following as the -i64 version:

<compiles fine>

(bind-type i64list <i64,i64list*>)

(bind-func cons-i64

  (lambda (a:i64 b:i64list*)

    (let ((pair:i64list* (zone-alloc)))

      (tset! pair 0 a)

      (tset! pair 1 b)

      pair)))

(bind-func car-i64

  (lambda (a:i64list*)

    (tref a 0)))

(bind-func cdr-i64

  (lambda (a:i64list*)

    (tref a 1)))

</compiles fine>

(bind-func sum-i64:[i64,i64list*]*

  (lambda (lst:i64list) 

    ( cond

        ((eqv? (cdr-i64 lst) ()) (car-i64 lst))

        (#t 

         (sum-i64 (cons-i64

                        (+ (car-i64 lst) (car-i64 (cdr-i64 lst)))

                        (cdr-i64 (cdr-i64 lst))

                )

              )

        )

    )

))

^^ compiling sum-i64 results in:
Type Error got NIL, was expecting NIL

What is the proper way to approach finding out which symbol the compiler is complaining about when it outputs NIL?