How to address the appointment table ?

I wrote an add-on plugin for A+ which generates invoices.
The add-on made use of $appointments->app_table to get the name of the appointment table in the DB and construct a SQL query.
Everything worked well till last update of A+.
Now $appointments->app_table returns an empty string and the query doesn't work any more.
Can you help me in solving the problems ?
Can I still use $appointments->app_table ?
Thanks for yor help
Kind regards