SQLContainer filtering not working for me

Hi! I hope if you could help me with this!
I have created a SQLContainer that has information related to countries (it points to a table in my database). I have for example an item that contains its primary key “code” with “ARG” value.
If I do this it doesn’t work:
container.addContainerFilter(new Equal(“code”, “ARG”));
Object toReturn = container.firstItemId();
It prints return=null.
If I try to print item Ids from container (before applying the filter) it works ok and prints all country codes in the table.
Do you know why filtering is not working?
Thanks in advance…