title: PHPExcel sheet index out of bounds exception
body: <p>I'm using PHPExcel to read an xlsx workbook, the following is the code I'm using so far just to set the active sheet </p>
  
  <pre><code>date_default_timezone_set('Europe/London');
  set_include_path(get_include_path() . PATH_SEPARATOR . './Classes/');
  
  include 'PHPExcel/IOFactory.php';
  
  $reader = PHPExcel_IOFactory::createReader('Excel2007');
  $reader-&gt;setReadDataOnly(true);
  $xlWB = $reader-&gt;load('/' . $target_file);
  $xlWB-&gt;setActiveSheetIndex(0); //EXCEPTION OCCURS HERE
  </code></pre>
  
  <p>but I'm getting an exception:</p>
  
  <blockquote>
    <p><strong>Fatal error</strong>: Uncaught exception 'PHPExcel_Exception' with message 'You tried to set a sheet active by the out of bounds index: 0. The actual number of sheets is 0.' in /[my root]/Classes/PHPExcel.php:695 Stack trace: #0 /[my-root]/conversion-demo.php(211): PHPExcel->setActiveSheetIndex(0) #1 {main} thrown in <strong>/[my root]/Classes/PHPExcel.php</strong> on line <strong>695</strong></p>
  </blockquote>
  
  <p>The file is an .xlsx file, it does exist (the path is correct) and there are 3 sheets in the file.</p>
  
  <p>Am I doing something fundamentally wrong here? </p>