[firebase-br] Erro constrain

[Levy Moreira]

TEnho a seguinte tabela

CREATE TABLE SNGPCPRE(
  NUMREG CHAR(10) NOT NULL,
  TIPREG CHAR(4) NOT NULL,
  UFCONS CHAR(2) NOT NULL,
  NOMPRE VARCHAR(60) NOT NULL);
ALTER TABLE SNGPCPRE ADD CONSTRAINT PK_SNGPCPRE PRIMARY KEY
  (NUMREG, TIPREG );

Preciso adicionar o seguinte constrain:

ALTER TABLE SNGPCPRE ADD CONSTRAINT PK_SNGPCPRE PRIMARY KEY
  (UFCONS );

Mas ele retorna o seguinte erro:

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".

hELP!!!

-- 
Levy Moreira