title: Unable to merge coverage data using genhtml
body: <p>I'm using genhtml (in Windows under cygwin) to generate a unit test coverage report from 2 coverage data files.  They are all coverage data from the same unit test source file.</p>
  
  <p>However, when I entered the following command:</p>
  
  <pre><code>perl genhtml /home/administrator/coverage1.dat /home/administrator/coverage2.dat
  </code></pre>
  
  <p>I got the following output and problem:</p>
  
  <pre><code>Reading data file /home/administrator/coverage1.dat
  Reading data file /home/administrator/coverage2.dat
  genhtml: Can't use an undefined value as a HASH reference at genhtml line 1506.
  </code></pre>
  
  <p>And it failed to generate anything.</p>
  
  <p>I played around with the files, and discovered that one reason might be because of the repeating headers and files in the coverage data.</p>
  
  <p>Is this a known issue, or does anyone know a way to get around it, or am I on the wrong track?</p>
  
  <p>Thanks.</p>