smf_logonline and who`is online list

[fabio]

hello

i have an installation of smf 1.1 rc2 and some other non smf pages

in each non smf page i use smf_logonline('pagename') with a corresponding addition to the lang files

when a user accesses such a page thn it appears correctly in who`s online
a secs later  the user is doing an "unknown action"

i checked the database , the table smf_log_online
in the url field when the online list is correct reads :
a:2:{s:10:"USER_AGENT";s:87:"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.12) Gecko/20050915 Firefox/1.0.7";s:6:"action";s:13:"custom_action";}

then a few seconds later
a:3:{s:4:"name";s:14:"error_docs/404";s:10:"USER_AGENT";s:87:"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.12) Gecko/20050915 Firefox/1.0.7";s:6:"action";s:13:"custom_action";}

what i do wrong?