diff --git a/protocol/src/lib.rs b/protocol/src/lib.rs
index e0d43da..c1c301c 100644
--- a/protocol/src/lib.rs
+++ b/protocol/src/lib.rs
@@ -980,8 +980,8 @@ impl fmt::Display for ProtocolError {
 #[cfg(feature = "async")]
 pub struct AsyncProtocol<R, W>
 where
-    R: AsyncRead + Unpin,
-    W: AsyncWrite + Unpin,
+    R: AsyncRead + Unpin + Send,
+    W: AsyncWrite + Unpin + Send,
 {
     reader: AsyncProtocolReader<R>,
     writer: AsyncProtocolWriter<W>,
@@ -990,8 +990,8 @@ where
 #[cfg(feature = "async")]
 impl<R, W> AsyncProtocol<R, W>
 where
-    R: AsyncRead + Unpin,
-    W: AsyncWrite + Unpin,
+    R: AsyncRead + Unpin + Send,
+    W: AsyncWrite + Unpin + Send,
 {
     /// New protocol session which completes the initial handshake and returns a handler.
     ///
@@ -1115,7 +1115,7 @@ enum DecryptState {
 #[cfg(feature = "async")]
 pub struct AsyncProtocolReader<R>
 where
-    R: AsyncRead + Unpin,
+    R: AsyncRead + Unpin + Send,
 {
     buffer: R,
     packet_reader: PacketReader,
@@ -1125,7 +1125,7 @@ where
 #[cfg(feature = "async")]
 impl<R> AsyncProtocolReader<R>
 where
-    R: AsyncRead + Unpin,
+    R: AsyncRead + Unpin + Send,
 {
     /// Decrypt contents of received packet from buffer.
     ///
@@ -1162,7 +1162,7 @@ where
 #[cfg(feature = "async")]
 pub struct AsyncProtocolWriter<W>
 where
-    W: AsyncWrite + Unpin,
+    W: AsyncWrite + Unpin + Send,
 {
     buffer: W,
     packet_writer: PacketWriter,
@@ -1171,7 +1171,7 @@ where
 #[cfg(feature = "async")]
 impl<W> AsyncProtocolWriter<W>
 where
-    W: AsyncWrite + Unpin,
+    W: AsyncWrite + Unpin + Send,
 {
     /// Encrypt contents and write packet buffer.
     ///