[plt-scheme] ffi: cant define (_ptr o _int)

With ffi I cant seem to do

(define output-int (_ptr o _int)) or anything like that. Why is this?
This fails:

(define output (_ptr o _int))
(define blah (get-ffi-obj "blah" lib (_fun output -> _int)))
(blah)

With
Welcome to MzScheme version 350, Copyright (c) 2004-2006 PLT Scheme Inc.
ffi:blah: expects 1 argument, given 0

So I tried:
(define (output) (_ptr o _int))
(define blah (get-ffi-obj "blah" lib (_fun (output) -> _int)))
(blah)

But that also fails.