title: Sending 'void' to parameter of incompatible type 'id'
body: <p>In Objective C, I have:</p>
  
  <pre><code>NSMutableArray *retVal = [[NSMutableArray alloc]initWithCapacity:1];
  NSMutableString *justTest = [[NSMutableString alloc]initWithString:@"hello"];
  unsigned char ch = //anything
  [retVal insertObject:[justTest appendFormat:@"%hhu", ch] atIndex:0]; //error here
  </code></pre>
  
  <p>X Code 5.1.1 gives me an error in the 4th line(as mentioned as comment) as <code>Sending 'void' to parameter of incompatible type 'id'</code></p>
  
  <p>What am I doing wrong here? Any help is appreciated.</p>