diff --git a/src/lib.rs b/src/lib.rs
index 47a95a8..05c263f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -30,7 +30,7 @@ pub enum Parent {
 unsafe impl Send for Parent {}
 
 pub trait WindowHandler {
-    type Message;
+    type Message: Send;
 
     fn on_frame(&mut self);
     fn on_event(&mut self, window: &mut Window, event: Event);