diff --git a/errors.go b/errors.go
index f2960c0..3b0af6e 100644
--- a/errors.go
+++ b/errors.go
@@ -12,6 +12,7 @@ var (
 	ErrRecvWindowFull        = NewGenericError("receive window full", false, true)
 	ErrNotHandshake          = NewGenericError("first packet is not handshake packet", false, true)
 	ErrConnClosed            = NewGenericError("connection closed", false, false)
+	ErrConnNotFound          = NewGenericError("connection not found", false, false)
 )
 
 type GenericError struct {
diff --git a/session.go b/session.go
index 6c55fc4..8b2a6e7 100644
--- a/session.go
+++ b/session.go
@@ -296,6 +296,8 @@ func (session *Session) ReceiveWith(localClientID, remoteClientID string, buf []
 
 		if conn, ok := session.connections[connKey(localClientID, remoteClientID)]; ok {
 			conn.SendAck(packet.SequenceId)
+		} else {
+			return ErrConnNotFound
 		}
 	}