Can get online users but not registered users - XEP-0133

I'm using 3.8.1 and XMPPFramework on iOS.

Trying to implement some adhoc commands using XEP-0133: http://xmpp.org/extensions/xep-0133.html#get-registered-users-num

I get an 'item-not-found' error when querying for either number of registered users or list of registered users:

<iq xmlns="jabber:client" type="error" id="get-registered-users-list-1" from="54.214.23.102" to="admin@54.214.23.102/2356fa20"><command xmlns="http://jabber.org/protocol/commands" action="execute" node="http://jabber.org/protocol/admin#get-registered-users-list">

</command>

<error code="404" type="cancel">

<item-not-found xmlns="urn:ietf:params:xml:ns:xmpp-stanzas">

</item-not-found>

</error>

</iq>

I know my XML is well-formed because I'm getting a response when querying for number of online users (using http://xmpp.org/extensions/xep-0133.html#get-online-users-num).

I'm querying from an admin account.

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Do I need to setup something on my server? Or does Openfire support online users but not registered users?

Is there another way to get the list of registered users?

I was thinking of adding every user that signs up to the admin account roster. Then I can just query the roster. If the number of users reaches into the millions will this be an eventual problem?

Also, I'm using MySQL for persistence. Can't I just query the registered users table (or whichever) in MySQL directly? I'm new to all backend stuff, so I haven't been able to easily explore this option yet.