Problem inserting unique MySQL record

I have a table that has a unique field, a record of this table has been deleted and now I want to insert it again, however MySQL generates the error: "Error Code: 1062. Duplicate entry '' for key ' '". How can I solve this?

CREATE TABLE:

CREATE TABLE 'bem' (
  'id' int(11) unsigned NOT NULL AUTO_INCREMENT,
  'cpf' varchar(17) NOT NULL,
  ...
  PRIMARY KEY ('id'),
  UNIQUE KEY 'cpf' ('cpf')
) ENGINE=InnoDB AUTO_INCREMENT=1764 DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC;

INSERT EXAMPLE:

INSERT INTO 'bem' (cpf, ...)VALUES ('12312312312', ...);