I can't prepare the following sql statement on employee.fdb:

update EMPLOYEE set LAST_NAME='XX'
where ((LAST_NAME=:old_last_name1) or
((:old_last_name2 is null) and (LAST_NAME is null)))


I get -804 Sql error code and Data type unknown message. I tried
FIBPlus and IBX, both failed.
I found that the problematic part is "(:old_last_name2 is null)"
expression.
If I remove that there's no trouble. What can I do?


Thanks in advance:
Tamas Varga