diff --git a/src/codec/mod.rs b/src/codec/mod.rs
index 9dc7d0c..922e08c 100644
--- a/src/codec/mod.rs
+++ b/src/codec/mod.rs
@@ -3,7 +3,7 @@ mod codec_stack;
 
 pub use self::framed::{Framed, usize_to_array, array_to_usize};
 
-pub trait Codec<In: ?Sized, Out: ?Sized> {
+pub trait Codec<In: ?Sized, Out: Sized> {
     fn decode(&mut self, buf: &mut In) -> Vec<Out>;
     fn encode(&self, a: Out) -> In;
 }