diff --git a/ixwebsocket/IXWebSocketTransport.cpp b/ixwebsocket/IXWebSocketTransport.cpp
index f9d36c52..23072f34 100644
--- a/ixwebsocket/IXWebSocketTransport.cpp
+++ b/ixwebsocket/IXWebSocketTransport.cpp
@@ -205,6 +205,12 @@ namespace ix
 
     void WebSocketTransport::setReadyState(ReadyState readyState)
     {
+        // Lock the _setReadyStateMutex for the duration of this
+        // method. This ensures that only a single thread runs the
+        // logic below avoiding concurrent execution of the
+        // close callback my multiple threads at the same time.
+        std::lock_guard<std::mutex> lock(_setReadyStateMutex);
+
         // No state change, return
         if (_readyState == readyState) return;
 
diff --git a/ixwebsocket/IXWebSocketTransport.h b/ixwebsocket/IXWebSocketTransport.h
index 8473c55c..97cfc15a 100644
--- a/ixwebsocket/IXWebSocketTransport.h
+++ b/ixwebsocket/IXWebSocketTransport.h
@@ -185,6 +185,8 @@ namespace ix
 
         // Hold the state of the connection (OPEN, CLOSED, etc...)
         std::atomic<ReadyState> _readyState;
+        // Mutex to serialize setReadyState() execution.
+        std::mutex _setReadyStateMutex;
 
         OnCloseCallback _onCloseCallback;
         std::string _closeReason;