title: Convert.FromBase64String(...) throws a FormatException
body: <p>The following line of code runs fine in <strong>IIS Express</strong>:</p>
  
  <pre><code>Convert.FromBase64String("dmVoaWNsZUlkPTE0MTM=??");
  </code></pre>
  
  <p>But when run on my local <strong>IIS 8 server</strong>, it throws the following exception:</p>
  
  <blockquote>
    <p><em><strong>System.FormatException:</strong>  The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.</em></p>
  </blockquote>
  
  <p>Why is this happening?</p>