title: Check calls Received() for async method
body: <p>When I run the following code:</p>
  
  <pre><code>[Test]
  public async Task Can_Test_Update()
  {
      var response = await _controller.UpdateAsync(Guid.NewGuid());
      response.Valid.Should().BeTrue();
  
      _commands.Received().UpdateAsync(
          Arg.Is&lt;Something&gt;(
              l =&gt; l.Status == Status.Updated)); 
  }
  </code></pre>
  
  <p>If I add "<code>await</code>" preceding the "<code>_commands.Received().UpdateAsync</code>", it throws a null reference exception.  How can I stop this happening, or is <code>await</code> not necessary?</p>