diff --git a/src/lib.rs b/src/lib.rs
index 3b6287c..b334408 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -25,7 +25,7 @@ mod coreprovider {
 use coreprovider::*;
 
 /// The empty type for cases which can't occur.
-#[derive(Copy)]
+#[derive(Copy, Eq, Ord)]
 pub enum Void { }
 
 impl Clone for Void {