diff --git a/Example/Program.cs b/Example/Program.cs
index 137f0e4f..d1f25694 100644
--- a/Example/Program.cs
+++ b/Example/Program.cs
@@ -73,8 +73,8 @@ namespace Example {
 
       ThreadPool.QueueUserWorkItem(notifyMsg);
 
-      using (var ws = new WebSocket("ws://echo.websocket.org", "echo"))
-      //using (var ws = new WebSocket("wss://echo.websocket.org", "echo"))
+      using (var ws = new WebSocket("ws://echo.websocket.org"))
+      //using (var ws = new WebSocket("wss://echo.websocket.org"))
       //using (var ws = new WebSocket("ws://localhost:4649"))
       //using (var ws = new WebSocket("ws://localhost:4649/Echo"))
       //using (var ws = new WebSocket("wss://localhost:4649/Echo"))
diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index dcd82a66..479a3adb 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -258,6 +258,16 @@ namespace WebSocketSharp
              : null;
     }
 
+    internal static string CheckIfValidProtocols (this string [] protocols)
+    {
+      return protocols.Contains (
+               protocol => protocol.Length == 0 || !protocol.IsToken ())
+             ? "Contains an invalid value."
+             : protocols.ContainsTwice ()
+               ? "Contains a value twice."
+               : null;
+    }
+
     internal static string CheckIfValidSendData (this byte [] data)
     {
       return data == null
@@ -334,6 +344,36 @@ namespace WebSocketSharp
              : stream.ToByteArray ();
     }
 
+    internal static bool Contains<T> (
+      this IEnumerable<T> source, Func<T, bool> comparer)
+    {
+      foreach (T value in source)
+        if (comparer (value))
+          return true;
+
+      return false;
+    }
+
+    internal static bool ContainsTwice (this string [] values)
+    {
+      var len = values.Length;
+
+      Func<int, bool> contains = null;
+      contains = index => {
+        if (index < len - 1) {
+          for (var i = index + 1; i < len; i++)
+            if (values [i] == values [index])
+              return true;
+
+          return contains (++index);
+        }
+
+        return false;
+      };
+
+      return contains (0);
+    }
+
     internal static T [] Copy<T> (this T [] src, long length)
     {
       var dest = new T [length];
diff --git a/websocket-sharp/WebSocket.cs b/websocket-sharp/WebSocket.cs
index 754c57fd..4bf27827 100644
--- a/websocket-sharp/WebSocket.cs
+++ b/websocket-sharp/WebSocket.cs
@@ -38,7 +38,6 @@ using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Diagnostics;
 using System.IO;
-using System.Linq;
 using System.Net.Sockets;
 using System.Net.Security;
 using System.Security.Cryptography;
@@ -89,7 +88,7 @@ namespace WebSocketSharp
     private string                  _origin;
     private bool                    _preAuth;
     private string                  _protocol;
-    private string                  _protocols;
+    private string []               _protocols;
     private volatile WebSocketState _readyState;
     private AutoResetEvent          _receivePong;
     private bool                    _secure;
@@ -148,10 +147,19 @@ namespace WebSocketSharp
     /// </param>
     /// <param name="protocols">
     /// An array of <see cref="string"/> that contains the WebSocket subprotocols
-    /// if any.
+    /// if any. Each value of <paramref name="protocols"/> must be a token defined
+    /// in <see href="http://tools.ietf.org/html/rfc2616#section-2.2">RFC 2616</see>.
     /// </param>
     /// <exception cref="ArgumentException">
-    /// <paramref name="url"/> is invalid.
+    ///   <para>
+    ///   <paramref name="url"/> is invalid.
+    ///   </para>
+    ///   <para>
+    ///   -or-
+    ///   </para>
+    ///   <para>
+    ///   <paramref name="protocols"/> is invalid.
+    ///   </para>
     /// </exception>
     /// <exception cref="ArgumentNullException">
     /// <paramref name="url"/> is <see langword="null"/>.
@@ -165,7 +173,13 @@ namespace WebSocketSharp
       if (!url.TryCreateWebSocketUri (out _uri, out msg))
         throw new ArgumentException (msg, "url");
 
-      _protocols = protocols.ToString (", ");
+      if (protocols != null && protocols.Length > 0) {
+        msg = protocols.CheckIfValidProtocols ();
+        if (msg != null)
+          throw new ArgumentException (msg, "protocols");
+
+        _protocols = protocols;
+      }
 
       _base64Key = CreateBase64Key ();
       _client = true;
@@ -677,21 +691,22 @@ namespace WebSocketSharp
     private string checkIfValidHandshakeResponse (HandshakeResponse response)
     {
       var headers = response.Headers;
-
-      string accept, version;
       return response.IsUnauthorized
              ? String.Format (
-                 "An HTTP {0} authorization is required.",
+                 "HTTP {0} authorization is required.",
                  response.AuthChallenge.Scheme)
              : !response.IsWebSocketResponse
-               ? "Not WebSocket connection response to the connection request."
-               : (accept = headers ["Sec-WebSocket-Accept"]) == null ||
-                 accept != CreateResponseKey (_base64Key)
+               ? "Not WebSocket connection response."
+               : !validateSecWebSocketAcceptHeader (
+                   headers ["Sec-WebSocket-Accept"])
                  ? "Invalid Sec-WebSocket-Accept header."
-                 : (version = headers ["Sec-WebSocket-Version"]) != null &&
-                   version != _version
-                   ? "Invalid Sec-WebSocket-Version header."
-                   : null;
+                 : !validateSecWebSocketProtocolHeader (
+                     headers ["Sec-WebSocket-Protocol"])
+                   ? "Invalid Sec-WebSocket-Protocol header."
+                   : !validateSecWebSocketVersionHeader (
+                       headers ["Sec-WebSocket-Version"])
+                     ? "Invalid Sec-WebSocket-Version header."
+                     : null;
     }
 
     private void close (CloseStatusCode code, string reason, bool wait)
@@ -894,8 +909,8 @@ namespace WebSocketSharp
 
       headers ["Sec-WebSocket-Key"] = _base64Key;
 
-      if (!_protocols.IsNullOrEmpty ())
-        headers ["Sec-WebSocket-Protocol"] = _protocols;
+      if (_protocols != null)
+        headers ["Sec-WebSocket-Protocol"] = _protocols.ToString (", ");
 
       var extensions = createExtensionsRequest ();
       if (extensions.Length > 0)
@@ -955,21 +970,17 @@ namespace WebSocketSharp
     {
       setClientStream ();
       var res = sendHandshakeRequest ();
-      var err = checkIfValidHandshakeResponse (res);
-      if (err != null) {
-        _logger.Error (err);
+      var msg = checkIfValidHandshakeResponse (res);
+      if (msg != null) {
+        _logger.Error (msg);
 
-        var msg = "An error has occurred while connecting.";
+        msg = "An error has occurred while connecting.";
         error (msg);
         close (CloseStatusCode.ABNORMAL, msg, false);
 
         return false;
       }
 
-      var protocol = res.Headers ["Sec-WebSocket-Protocol"];
-      if (!protocol.IsNullOrEmpty ())
-        _protocol = protocol;
-
       processRespondedExtensions (res.Headers ["Sec-WebSocket-Extensions"]);
 
       var cookies = res.Cookies;
@@ -1364,6 +1375,32 @@ namespace WebSocketSharp
              host == expected;
     }
 
+    // As client
+    private bool validateSecWebSocketAcceptHeader (string value)
+    {
+      return value != null && value == CreateResponseKey (_base64Key);
+    }
+
+    // As client
+    private bool validateSecWebSocketProtocolHeader (string value)
+    {
+      if (value == null)
+        return _protocols == null;
+
+      if (_protocols == null ||
+          !_protocols.Contains (protocol => protocol == value))
+        return false;
+
+      _protocol = value;
+      return true;
+    }
+
+    // As client
+    private bool validateSecWebSocketVersionHeader (string value)
+    {
+      return value == null || value == _version;
+    }
+
     #endregion
 
     #region Internal Methods