diff --git a/websocket-sharp/Server/WebSocketSessionManager.cs b/websocket-sharp/Server/WebSocketSessionManager.cs
index 8b2f7cd3..188db488 100644
--- a/websocket-sharp/Server/WebSocketSessionManager.cs
+++ b/websocket-sharp/Server/WebSocketSessionManager.cs
@@ -280,13 +280,24 @@ namespace WebSocketSharp.Server
         return _waitTime;
       }
 
-      internal set {
-        if (value == _waitTime)
+      set {
+        if (value <= TimeSpan.Zero)
+          throw new ArgumentOutOfRangeException ("value", "Zero or less.");
+
+        string msg;
+        if (!canSet (out msg)) {
+          _log.Warn (msg);
           return;
+        }
 
-        _waitTime = value;
-        foreach (var session in Sessions)
-          session.Context.WebSocket.WaitTime = value;
+        lock (_sync) {
+          if (!canSet (out msg)) {
+            _log.Warn (msg);
+            return;
+          }
+
+          _waitTime = value;
+        }
       }
     }