C# NullReferenceException when accessing lock object

I'm wondering if anyone of you can help me with this scenario. I have a multithreaded program that uses client server socket connections.

        public static object _mLock = new object();

        private void OnReceive_FromServer(IAsyncResult iar)
        {
            lock (_mLock)
            {
                try {
                     // some code
                catch(Exception e) {
                     Log(e);
                }
            }
        }

In this method I get NullReferenceException on _mLock public static field for some reason I can't explain. Can anyone of you explain how and what object can apparently be null here on line 5?

I've tried to search the internet for this particular problem but I get thousands of results not having a similar scenario...