Why there is no warning or even compilation error?

AwaitAndElvis.cs

using System.Threading.Tasks;

namespace AwaitAndElvis
{
    class Program
    {
        static void Main(string[] args)
        {
            Run(null).Wait();
        }

        private static async Task Run(Program p)
        {
            var a = await p?.Async(); // <- NRE
        }

        public async Task<int> Async()
        {
            return 5;
        }
    }
}