title: no implicit conversion of nil into String
body: <p>currently I'm trying to set up a simple app in Ruby on Rails. I've migrated the db, seeded it and am now stuck at the following error.</p>
  
  <pre><code>no implicit conversion of nil into String
  
  Extracted source (around line #4):
  
  1: &lt;!--[if lt IE 9]&gt;
  2: &lt;script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"&gt;&lt;/script&gt;
  3: &lt;![endif]--&gt;
  4: &lt;%= stylesheet_link_tag 'blueprint/screen', :media =&gt; 'screen' %&gt;
  5: &lt;%= stylesheet_link_tag 'blueprint/print',  :media =&gt; 'print' %&gt;
  6: &lt;%= stylesheet_link_tag 'custom', :media =&gt; 'screen' %&gt;
  </code></pre>
  
  <p>It seems like the asset tags helpers are not working properly or returning nil (if that's even possible)?</p>