diff --git a/src/connection/codec.rs b/src/connection/codec.rs
index 2f657ae..6529d3d 100644
--- a/src/connection/codec.rs
+++ b/src/connection/codec.rs
@@ -51,7 +51,7 @@ impl Codec for APCodec {
 
         self.encode_cipher.encrypt(&mut buf[offset..]);
 
-        let mac = [0u8; MAC_SIZE];
+        let mut mac = [0u8; MAC_SIZE];
         self.encode_cipher.finish(&mut mac);
         buf.extend_from_slice(&mac);