diff --git a/viz-core/src/handler/after.rs b/viz-core/src/handler/after.rs
index 0ff25ba1..a49f299f 100644
--- a/viz-core/src/handler/after.rs
+++ b/viz-core/src/handler/after.rs
@@ -20,6 +20,7 @@ impl<H, F, I, O> Handler<I> for After<H, F>
 where
     I: Send + 'static,
     H: Handler<I, Output = Result<O>> + Clone,
+    O: Send + 'static,
     F: Handler<Result<O>, Output = Result<O>> + Clone,
 {
     type Output = F::Output;