"invalid argument" calling sane_open in Rust through FFI

Question

I'm trying to set up a Rust FFI for libsane, the Linux scanning library. Executing the sane_open function returns the error code 4 indicating an invalid argument. I believe it has something to do with my parameter declaration.

// If successful, places a new handle in *h
// SANE_String_Const is const char* and SANE_Handle is void*
SANE_Status sane_open(SANE_String_Const devname, SANE_Handle *h);

Bindgen generates the following:

pub type SANE_String_Const = *const ::std::os::raw::c_char
pub type SANE_Handle = *mut ::std::os::raw::c_void
pub fn sane_open(devname: SANE_String_Const, handle: *mut SANE_Handle) -> SANE_Status;

Here is what I have done:

pub fn test_open() {
    unsafe {
        let mut version_code = 0;
        let result = sane_init(&mut version_code, None);
        assert_eq!(result, SANE_Status_SANE_STATUS_GOOD);

        let mut handle: SANE_Handle = std::ptr::null_mut();
        let dev = CString::new("net1;dev0").unwrap();
        let result = sane_open(dev.as_ptr(), &mut handle);
        assert_eq!(result, SANE_Status_SANE_STATUS_GOOD);

        sane_close(handle);
        sane_exit();
    }
}

Note that bindgen generates an Option<_> type for the callback where None means passing NULL. My minimal working example in C works flawlessly:

/* Link with -l sane */
#include <sane/sane.h>
#include <stdio.h>
#include <assert.h>

int main() {
    SANE_Int version_code = 0;
    assert(sane_init(&version_code, NULL) == SANE_STATUS_GOOD);

    SANE_Handle h;
    SANE_Status r = sane_open("net1;dev0", &h);
    printf("Result: %d\n", r);
    if(r == 0)
        sane_close(h);
    sane_exit();
}

What's wrong? Why does my Rust code behave differently?