title: SIGABRT when using NSDictionary setObject: forKey:
body: <p>I'm using a property-file with this structure:</p>
  
  <pre><code>maxRating     Number    --&gt; 5
  gradePercent  Dictonary
  &gt;grade1       Number    --&gt; 0.85
  &gt;grade2       Number    --&gt; 0.70
  &gt;grade3       Number    --&gt; 0.55
  &gt;grade4       Number    --&gt; 0.40
  </code></pre>
  
  <p>To read the properties I'm using</p>
  
  <pre><code>NSDictionary *plist = [[NSDictionary alloc] initWithContentsOfFile:filepath];
  
  rating = [[plist objectForKey:@"maxRating"] intValue];
  gradePercent = (NSMutableDictionary*)[[plist objectForKey:@"gradePercent"] copy];
  </code></pre>
  
  <p>Till here every thing works perfectly...I can get the right objects with <code>[gradePercent objectForKey:@"grade1"]</code></p>
  
  <p>but when I try to set one of them with</p>
  
  <pre><code>[gradePercent setObject:[NSNumber numberWithFloat:0.90] forKey:@"grade1"]
  </code></pre>
  
  <p>I always get a SIGABRT error and the APP is crashing.</p>
  
  <p>Does anyone see why this isn't working? Because I don't -.-</p>