Item-forbidden error posting items to Publish-Subscribe node: XEP-0060 specs violated?

Hi,

I was trying to set up an XMPP Publish-Subscribe node and to configure it such that when new items are posted to the node, the payload gets automatically delivered to all the subscribers. I tried to configure the node like this:

<pubsubxmlns='http://jabber.org/protocol/pubsub#owner'>
    <configurenode='mynode'>
        <xxmlns='jabber:x:data'type='submit'>
            <fieldvar='pubsub#persist_items'>
                <value>true</value>
            </field>
            <fieldvar='pubsub#deliver_payloads'>
                <value>true</value>
            </field>
        </x>
    </configure>
</pubsub>

But if I then try to post an item to the node, I get an item-forbidden error:

<errorcode="400"type="modify">
    <bad-requestxmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
    <item-forbiddenxmlns="http://jabber.org/protocol/pubsub#errors"/>
</error>

However, if instead of using "true" I use "1" in the configuration, I get no errors:

<pubsubxmlns='http://jabber.org/protocol/pubsub#owner'>
        <configurenode='mynode'>
                 <xxmlns='jabber:x:data'type='submit'>
                          <fieldvar='pubsub#persist_items'>
                                   <value>1</value>
                          </field>
                          <fieldvar='pubsub#deliver_payloads'>
                                   <value>1</value>
                          </field>
                 </x>

    </configure>

</pubsub>

XEP-0060 clarifies that "implementations MUST support both styles of lexical representation" xmpp.org/extensions/xep-0060.html#nt-id183715.

Why am I getting an "item-forbidden" error if I user "true" in the configuration?

Thanks