using RDB$Set_Context and RDB$Get_Context on Firebird

[F10Software]

I'm still using version 2.20 of your DBX component for Firebird. Then, I do:

select rdb$set_context('USER_SESSION', 'USER_ID', 10) from rdb$database
it seems to work.

but when a try to get its value using
select rdb$get_context('USER_SESSION', 'USER_ID') from rdb$database

then I receive a
"function RDB$GET_CONTEXT could not be matched" error message

I'm using Firebird 2.1, which accepts these context variables. I can do these selects on IBExpert and they work fine.

where is the problem ? Is this a version problem ? On version 2.50 does this problem occurs too ?

thanks

Marcos