locked section misfire

Greetingz,

take a look at the code at the bottom, which is being used by a multithreaded WebApplication.
Resharper says "Expression is always null", now consider 2 threads entering this code. 1st thread creates new container, while 2nd will only check if upon entering the locked section container is being created, so the /*XXX*/container= assignment will have another meaning.

ReSharperSetup.7.0.61.135

        private static IDictionary<decimal, object> Container
        {
            get
            {
                var key = "123"; 
                var container = HttpContext.Current.Application[key] as IDictionary<decimal, object>;
                if (container == null)
                {
                    lock (_syncContainer)
                    {
                        // Resharper: Destination has the same value before assignment                        
                        /*XXX*/ container = HttpContext.Current.Application[key] as IDictionary<decimal, object>; // Resharper: Expression is always null
                        if (container == null)
                        {
                            container = new ConcurrentDictionary<decimal, object>();
                            HttpContext.Current.Application[key] = container;
                        }
                    }
                }

                return container;
            }
        }