WPF mutex for single app instance not working

I'm trying to use the mutex method for only allowing one instance of my app to run. That is - I only want a max of one instance for all users on a machine. I've read through the various other threads on this issue and the solution seems straightforward enough but in testing I can't get my second instance to not run. Here is my code...

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        // check that there is only one instance of the control panel running...
        bool createdNew = true;
        using (Mutex instanceMutex = new Mutex(true, @"Global\ControlPanel", out createdNew))
        {
            if (!createdNew)
            {
                Application.Current.Shutdown();
                return;
            }
        }

        base.OnStartup(e);
    }
}