title: struct.error reported in pyinstaller
body: <p>I have a python program that failed with </p>
  
  <pre><code>struct.error: argument for 's' must be a string
  </code></pre>
  
  <p>I tracked down and found the following attempt got stuck:</p>
  
  <pre><code>struct.pack('!iiiibc30s', 48, 11718431L, 4190519, 10773240, 1, 'b', u'libmkl_core.dylib\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
  </code></pre>
  
  <p>I think this is a unicode issue, but not sure why this happen and how to fix this. The python program I used is "pyinstaller", and this happened when I was trying to build a single executable. TThe python version I was using is 2.7, and the system is mountain lion.</p>