Oracle Forms

Hello !
I have a problem: I've got an item b_item that is not included in the base table of the block (A) but in another table (B). However, it must be possible to use this item for query restrictions. The base table A (and the block) has a foreign key-column that refers to table B - let's call it table_b_fk. In the forms application, this column is not displayed.
So, after entering the query restrictions, i use the PRE-QUERY-Trigger to connect both tables like this:

if b_item is not null then
copy ('# in
(select table_b_pk from B
where b_item like '''|| name_in ( :blockname.b_item) || ''' ) ',
'blockname.table_b_fk');
end if;

This has worked out well in many other form applications. However, this time it doesn't work at all. This copy-statement causes error message FRM-50016 (Legal characters are 0-9 - + E .).
What must i do to make it work ?
Query length of item table_b_fk is 255 which should be enough.
If you got some ideas about it, please help.
Thanks a lot !
Yours,
Marc