Cannot call call_mut on an unboxed closure with no arguments due to type () not being ()

[huonw]

#![feature(unboxed_closures)]

use std::ops::FnMut;

fn main() {
    let mut zero = |&mut:| {}; zero.call_mut(());

    // let mut one = |&mut: _: int| {}; one.call_mut((1,));
}

<anon>:6:46: 6:48 error: mismatched types: expected `()` but found `()` (expected tuple but found ())
<anon>:6     let mut zero = |&mut:| {}; zero.call_mut(());
                                                      ^~

Swapping the comments compiles fine.

(At a guess the compiler is looking for a tuple with no fields, which is not the same as unit ().)