segmenation fault

[Catonano]

in using the guile FFI I am having a segmentation fault

This is the C unction that I'm trying to wrap

https://www.gaia-gis.it/gaia-sins/freexl-1.0.1-doxy-doc/html/freexl_8h.html#acbd27ba5bc7b21d4ae32c0542d51f1e4

My code is here
https://gitlab.com/humanitiesNerd/guile-freexl
(configure is to be called with --with-freexl-libdir=... otherwise it won't
work)

Here's an excerpt

(define freexl-open
  (let* ((ptr     (freexl-func "freexl_open"))
         (proc    (pointer->procedure int ptr '(* *)))
     ;;const char *path, const void **xls_handle
     )
  (lambda (path)
    (let ((handle (scm->pointer 0))
      (path-ptr (string->pointer path)))
      (proc path-ptr handle)))))


At the REPL, when calling it with
> (freexl-open"resources/Lavoro_P.xsl")

it produces a segmentation fault

probably this line
(scm->pointer 0)
is not correct

But I don't know how to produce a void pointer and then pass it to
freexl-open

Thanks in advance