Subject Firebird 4 and RDB$ERROR

I am using Firebird 4.0 Beta 1. The release notes describe a new system function RDB$ERROR:

"The function RDB$ERROR() takes a PSQL error context as input and returns the specific context of the active

exception. Its scope is confined to the context of the exception-handling block in PSQL. Outside the exception
handling block, RDB$ERROR always returns NULL"
 To test this in iSQL I have tried: select rdb$error(gdscode) from rdb$database

This results in:
Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown

I was expecting NULL to be returned. Have I misunderstood?