title: Objective-C category is not loaded - How to debug this
body: <p>I have two projects which the <a href="http://restkit.org" rel="noreferrer">RestKit framework</a>.</p>
  
  <p>One project works without problems but another project fails, as soon as the RestKit framework is used.</p>
  
  <p>I found out that the failing code is this:</p>
  
  <pre><code>return [anNSString MD5];
  </code></pre>
  
  <p>The <code>MD5</code> method is a category method and is imported like this:</p>
  
  <pre><code>#import "NSString+MD5.h"
  </code></pre>
  
  <p>However, in one project, I keep getting the following error:</p>
  
  <pre><code>-[__NSCFString MD5]: unrecognized selector sent to instance 0x88a3390
  </code></pre>
  
  <p>I understand the basics of categories, and that they can be loaded at runtime, but I don't see why the category does not get loaded in this case. </p>
  
  <p>These are the files on github: <a href="https://github.com/RestKit/RestKit/blob/master/Code/Network/NSString+MD5.m" rel="noreferrer">NSString+MD5.m</a>, <a href="https://github.com/RestKit/RestKit/blob/master/Code/Network/NSString+MD5.h" rel="noreferrer">NSString+MD5.h</a>, </p>