diff --git a/websocket-sharp/WebSocket.cs b/websocket-sharp/WebSocket.cs
index 9bc4ad98..e8256b37 100644
--- a/websocket-sharp/WebSocket.cs
+++ b/websocket-sharp/WebSocket.cs
@@ -3015,7 +3015,7 @@ namespace WebSocketSharp
 
       byte[] bytes;
       if (!data.TryGetUTF8EncodedBytes (out bytes))
-        throw new ArgumentException ("It could not be UTF8 encoded.", "data");
+        throw new ArgumentException ("It could not be UTF-8-encoded.", "data");
 
       sendAsync (Opcode.Text, new MemoryStream (bytes), completed);
     }