title: NSubstitute Error UnexpectedArgumentMatcherException
body: <p>I'm getting the following error:</p>
  
  <blockquote>
    <p>NSubstitute.Exceptions.UnexpectedArgumentMatcherException: 'Argument
    matchers (Arg.Is, Arg.Any) should only be used in place of member
    arguments. Do not use in a Returns() statement or anywhere else
    outside of a member call. Correct use:<br>
    sub.MyMethod(Arg.Any()).Returns("hi") Incorrect use:<br>
    sub.MyMethod("hi").Returns(Arg.Any())'</p>
  </blockquote>
  
  <p>When trying to mock out the following interface:</p>
  
  <pre><code>public interface IMyDate
  {
      DateTime GetDate();
  }
  </code></pre>
  
  <p>Here's where I mock it:</p>
  
  <pre><code>var myDate = Substitute.For&lt;IMyDate&gt;();
  myDate.GetDate().Returns(testDate); // Error thrown here
  </code></pre>
  
  <p>Please can anyone explain what I'm doing wrong?</p>
  
  <pre><code>var myDate = Substitute.For&lt;IMyDate&gt;();
  myDate.GetDate().Returns(new DateTime(2018, 04, 05)); // Error thrown here
  </code></pre>
  
  <p>Gives the same result.</p>