ArgumentNullException was unhandled, out of the blue
<p>I have been working on a project on and off, but I haven't touched it for 2 months, today I came back to it and I was unable to run it due to an error.</p>

<pre><code>IEnumerable&lt;MediaTitle&gt; query = mediaTitleCollection.OfType&lt;FilmMedia&gt;();
</code></pre>

<p>The debugger stops on this line and gives the error described in title along with </p>

<blockquote>
  <p>Value cannot be null. Parameter name: source</p>
</blockquote>

<p>Its very frustrating since the code was working 100% before and now its dominated by this error. I have tried to clean the solution without much luck.</p>
