Error: 'String' is not convertible to 'String!'
<pre><code>mapView.rac_valuesForKeyPath("userTrackingMode", observer: self).subscribeNextAs { 
// block handling
</code></pre>

<p>I get an error <code>'String' is not convertible to 'String!'</code>. Any suggestions what this may mean?</p>

<p>I used to think, that <code>String!</code> is same as <code>String</code>, so it is unwrapped <code>String?</code>...</p>

<ul>
<li>Xcode 7.3.1</li>
<li>Swift 2.2</li>
<li>ReactiveCocoa 4.1.0</li>
</ul>