title: TypeError: Cannot read property 'valid' of undefined
body: <p>I have the following textarea:</p>
  
  <pre><code>&lt;textarea class="form-control" [(ngModel)]="content" name="content" required&gt;
          &lt;/textarea&gt;
  </code></pre>
  
  <p>and the following submit button:</p>
  
  <pre><code> &lt;button type="submit" class="btn btn-default" [disabled]="content.valid"&gt;New comment&lt;/button&gt;
  </code></pre>
  
  <p>As I saw in the angular 2 form guide (<a href="https://angular.io/docs/ts/latest/guide/forms.html">https://angular.io/docs/ts/latest/guide/forms.html</a>) I can use the <code>x.valid</code> in order to check if it's not empty.</p>
  
  <p>Why do I get <code>TypeError: Cannot read property 'valid' of undefined</code> error?</p>