inserting a file in a module?

[Damien Mattei <damien.mattei@gmail.com>]

hello,
i have problem i do not understand when inserting a file in a module,
things i never done before:

(define-module (matrix)

  #:use-module (oop goops)

  #:use-module ((guile)
        #:select (*)
        #:prefix orig:)

  #:export (<matrix>
        matrix
        matrix-v)

  )

(define scheme-plus-path "/Users/mattei/Dropbox/git/Scheme-PLUS-for-Guile")

(include (string-append scheme-plus-path "/Scheme+.scm"))

...

i get this error:
scheme@(guile-user)> (use-modules (matrix))
;;; note: source file /usr/local/share/guile/site/3.0/matrix.scm
;;;       newer than compiled
/Users/mattei/.cache/guile/ccache/3.0-LE-8-4.6/usr/local/share/guile/site/3.0/matrix.scm.go
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling /usr/local/share/guile/site/3.0/matrix.scm
;;; WARNING: compilation of /usr/local/share/guile/site/3.0/matrix.scm failed:
;;; In procedure string-prefix?: Wrong type argument in position 2
(expecting string): (string-append scheme-plus-path "/Scheme+.scm")
While compiling expression:
In procedure string-prefix?: Wrong type argument in position 2
(expecting string): (string-append scheme-plus-path "/Scheme+.scm")

but it works if i replace the string-append by the string already appended :
(include "/Users/mattei/Dropbox/git/Scheme-PLUS-for-Guile/Scheme+.scm")