diff --git a/websocket-sharp/Server/WebSocketBehavior.cs b/websocket-sharp/Server/WebSocketBehavior.cs
index fbb37d72..d4ac13f3 100644
--- a/websocket-sharp/Server/WebSocketBehavior.cs
+++ b/websocket-sharp/Server/WebSocketBehavior.cs
@@ -356,12 +356,21 @@ namespace WebSocketSharp.Server
     ///   The default value is <see langword="null"/>.
     ///   </para>
     /// </value>
+    /// <exception cref="InvalidOperationException">
+    /// The set operation is not available when the session has already started.
+    /// </exception>
     public Func<string, bool> HostValidator {
       get {
         return _hostValidator;
       }
 
       set {
+        if (_websocket != null) {
+          var msg = "The set operation is not available.";
+
+          throw new InvalidOperationException (msg);
+        }
+
         _hostValidator = value;
       }
     }