Hi

I have two tables as follows.
An invoice table:

CREATE TABLE TBLINVOICE (
INVOICENO INTEGER NOT NULL,
SCHOOLID INTEGER,
SOID SMALLINT,
INVOICEDATE DATE,
PAID BOOLEAN /* BOOLEAN = CHAR(1) DEFAULT 'n' */,
DATEPAID DATE,
THEIRREFERENCE SHORTNOTES /* SHORTNOTES = VARCHAR(255) */,
NOTES SHORTNOTES /* SHORTNOTES = VARCHAR(255) */
);

ALTER TABLE TBLINVOICE ADD CONSTRAINT PK_TBLINVOICE PRIMARY KEY
(INVOICENO);

ALTER TABLE TBLINVOICE ADD CONSTRAINT FK_TBLINVOICE FOREIGN KEY
(SCHOOLID) REFERENCES TBLCUSTOMER (SCHOOLID) ON DELETE CASCADE ON
UPDATE CASCADE;

And a table for special offer details:

CREATE TABLE TBLSPECIALOFFERCODE (
SOID SMALLINT NOT NULL,
SPECIALOFFERCODE SHORTSTRINGS NOT NULL /* SHORTSTRINGS = CHAR
(50) */,
DETAILSFORINVOICE SHORTSTRINGS /* SHORTSTRINGS = CHAR(50) */,
DISCOUNT FLOAT,
AMAZONVOUCHERVALUE FLOAT,
DETAILS SHORTNOTES /* SHORTNOTES = VARCHAR(255) */
);

ALTER TABLE TBLSPECIALOFFERCODE ADD PRIMARY KEY (SOID);

When I try and add another foreign key to the tblInvoice using:
alter table TBLINVOICE
add constraint FK_TBLINVOICE
foreign key (SOID)
references TBLSPECIALOFFERCODE(SOID)
on delete CASCADE
on update CASCADE


I get error:

This operation is not defined for system tables.
unsuccessful metadata update.
STORE RDB$INDICES failed.
attempt to store duplicate value (visible to active transactions) in
unique index "RDB$INDEX_5".

I have no data in either table, so I'm not sure what is duplicate.

Thanks for the help

Pete