 <p>According to <a href="http://flask.pocoo.org/docs/api/#flask.url_for" rel="nofollow"><code>url_for</code> documentation</a>:</p> <blockquote> <p>If the value of a query argument is None, the whole pair is skipped.</p> </blockquote> <p>Make sure that <code>url_title</code> is not <code>None</code>.</p> <p>Or specify default value for <code>url_title</code> in the <code>artitle_page</code> function.</p>