title: Swift: Nil is incompatible with return type String
body: <p>I have this code in Swift:</p>
  
  <pre><code>guard let user = username else{
          return nil
      }
  </code></pre>
  
  <p>But I'm getting the following errors:</p>
  
  <pre><code>Nil is incompatible with return type String
  </code></pre>
  
  <p>Any of you knows why or how I return nil in this case?</p>
  
  <p>I'll really appreciate your help</p>