diff --git a/websocket-sharp/Server/WebSocketServiceHost`1.cs b/websocket-sharp/Server/WebSocketServiceHost`1.cs
index 037e7e78..2eb59e82 100644
--- a/websocket-sharp/Server/WebSocketServiceHost`1.cs
+++ b/websocket-sharp/Server/WebSocketServiceHost`1.cs
@@ -31,7 +31,7 @@ using System;
 namespace WebSocketSharp.Server
 {
   internal class WebSocketServiceHost<TBehavior> : WebSocketServiceHost
-    where TBehavior : WebSocketBehavior
+    where TBehavior : WebSocketBehavior, new ()
   {
     #region Private Fields