diff --git a/websocket-sharp/Server/WebSocketServer.cs b/websocket-sharp/Server/WebSocketServer.cs
index 9f29ea56..7b3ca995 100644
--- a/websocket-sharp/Server/WebSocketServer.cs
+++ b/websocket-sharp/Server/WebSocketServer.cs
@@ -558,13 +558,20 @@ namespace WebSocketSharp.Server
       }
 
       set {
-        var msg = _state.CheckIfAvailable (true, false, false);
-        if (msg != null) {
+        string msg;
+        if (!checkIfAvailable (true, false, false, true, out msg)) {
           _logger.Error (msg);
           return;
         }
 
-        _userCredFinder = value;
+        lock (_sync) {
+          if (!checkIfAvailable (true, false, false, true, out msg)) {
+            _logger.Error (msg);
+            return;
+          }
+
+          _userCredFinder = value;
+        }
       }
     }