(Apparent) bug: approx_equals always returns true if one input is empty?

For me, this test fails:

pub use derive_deftly;
use derive_deftly::{define_derive_deftly,Deftly};

define_derive_deftly! {
    pub Wobbling =

    impl $ttype {
	pub fn wobble_status(&self) -> &'static str {
	    ${if approx_equal({wobble}, {}) {
		"bug"
	    } else {
		"okay"
	    }}
	}
    }
}

#[derive(Deftly)]
#[derive_deftly(Wobbling)]
pub struct Wobbler {
}

#[test]
fn check() {
    assert_eq!(Wobbler {}.wobble_status(), "okay");
}

I would have expected that approx_equal({wobble}, {}) would always be false.