prosody stanza parse xml

>stanza.stanza("x"):add_child(xml.parse("<y/>"))
| Fatal error while running command, it did not complete
| Error: console:1: attempt to index global 'xml' (a nil value)

>stanza.stanza("x"):add_child(xml.parse("<y/>")):up()        
| Fatal error while running command, it did not complete
| Error: console:1: attempt to index global 'xml' (a nil value)

lua code:
message:tag("html", "http://jabber.org/protocol/xhtml-im"):add_child(xml.parse(post_body.html)):up();

error log:
Feb 14 14:29:54 http.server     error   Traceback[httpserver]: /usr/lib/prosody/util/stanza.lua:214: bad argument #1 to 'pairs' (table expected, got string)
stack traceback:
        [C]: in function 'pairs'
        /usr/lib/prosody/util/stanza.lua:214: in function 'self'
        /usr/lib/prosody/util/stanza.lua:231: in function '_dostring'
        /usr/lib/prosody/util/stanza.lua:241: in function </usr/lib/prosody/util/stanza.lua:239>
        [C]: in function 'tostring'
        /usr/lib/prosody/modules/mod_post_msg.lua:62: in function </usr/lib/prosody/modules/mod_post_msg.lua:37>
        (tail call): ?
        (tail call): ?
        /usr/lib/prosody/util/events.lua:67: in function 'fire_event'
        /usr/lib/prosody/net/http/server.lua:217: in function </usr/lib/prosody/net/http/server.lua:160>
        ...
        /usr/lib/prosody/net/http/server.lua:108: in function 'process_next'
        /usr/lib/prosody/net/http/server.lua:124: in function 'success_cb'
        /usr/lib/prosody/net/http/parser.lua:150: in function 'feed'
        /usr/lib/prosody/net/http/server.lua:146: in function </usr/lib/prosody/net/http/server.lua:145>
        (tail call): ?
        /usr/lib/prosody/net/server_select.lua:847: in function </usr/lib/prosody/net/server_select.lua:829>
        [C]: in function 'xpcall'
        /usr/bin/prosody:372: in function 'loop'
        /usr/bin/prosody:402: in main chunk
        [C]: ?