node getting formed but cannot publish in pubsub xmpp

Question

I am using libstrophe C library to Create xmpp clint.I am successfully able to create node(open node) and send request for subscription from other users,which get added. Now when i try to publish,using the following stanza,

<iq type='set'
    from='[email protected]/blogbot'
    to='pubsub.pratik.com'
    id='publish1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='pratik'>
      <item id='123456'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Soliloquy</title>
          <summary>
To be, or not to be
          </summary>
        </entry>
      </item>
    </publish>
  </pubsub>

I get,

<error code="501" type="cancel"><feature-not-implemented xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/></error>

Any suggestions? am using ejabberd as server