title: Cannot determine argument specifications to use
body: <p>I'm having a problem with NSubstitute.
  I have this short code:</p>
  
  <pre><code>ReportingCycleDeliveryRepository
  .When(f =&gt; f.Add(Arg.Any&lt;ReportingCycleDelivery&gt;()))
  .Do(x =&gt; RepCycleDeliveries.Add((ReportingCycleDelivery)x[0]));
  </code></pre>
  
  <p>So when my (void) method <code>ReportingCycleDeliveryRepository.Add()</code> is invoked with any ReportingCycleDelivery argument, it should add this item to my <code>RepCycleDeliveries</code> list.</p>
  
  <p>But instead, it throws an exception:</p>
  
  <p><em>NSubstitute.Exceptions.AmbiguousArgumentsException</em></p>
  
  <p>"Cannot determine argument specifications to use.
  Please use specifications for all arguments of the same type."</p>
  
  <p>Why is that? Why can't NSubstitute determine the correct argument specifications to use? I am clearly providing a hint, that the argument can be any <code>ReportingCycleDelivery</code> item.</p>