title: Cannot read property 'concat' of undefined using knockout
body: <p>I created a computed observable rather than having a extra code in the view and now when I run the page I get an error in the console stating: </p>
  
  <blockquote>
    <p>"Uncaught TypeError: Cannot read property 'concat' of undefined" </p>
  </blockquote>
  
  <p>Here is what I have so far:</p>
  
  <pre><code>self.email = ko.observable(initialData.userEmail || '');
  self.emailMailto = ko.computed(function(){ return 'mailto:'+self.email();});
  </code></pre>
  
  <p>I know that email observable is returning the appropriate data but I am not sure if it is referring to <code>emailMailto</code> as undefined. </p>
  
  <p>this is my view:</p>
  
  <pre><code>&lt;p&gt;&lt;a data-bind="attr:{href:emailMailto},text:email"&gt;&lt;/a&gt;&lt;/p&gt;  
  </code></pre>