TypeError: Cannot read property ‘valid’ of undefined

I have the following textarea:

<textarea class="form-control" [(ngModel)]="content" name="content" required>
        </textarea>

and the following submit button:

 <button type="submit" class="btn btn-default" [disabled]="content.valid">New comment</button>

As I saw in the angular 2 form guide (https://angular.io/docs/ts/latest/guide/forms.html) I can use the x.valid in order to check if it’s not empty.

Why do I get TypeError: Cannot read property 'valid' of undefined error?