I'm using Firebird 1.5.2
I try to issue the following command

alter table DIORTHOTIKO
add constraint FK_DIORTHOTIKO_1
foreign key (ETOS_PROT,SEIRA_PROT,AR_PROT)
references BOOK(ETOS,SEIRA,AR_PROT)


and I get the error message

This operation is not defined for system tables.
unsuccessful metadata update.
object BOOK is in use .

Only SYSDBA is connected to the database.

I have extracted metadata to a script from the database
and create a new database from the script.
Then I export the data from the first database
and import the data to the second database.

To the second database I can issue the command
alter table with no problem.

Any help why is this happening;