The nextId method doesn't have a good path for non-existing sequences when autocreate is not enabled.

    public function nextId($seqName, $onDemand = true)
    {
        $sequenceName = $this->conn->quoteIdentifier($this->conn->formatter->getSequenceName($seqName), true);

        $query = "SELECT NEXTVAL('" . $sequenceName . "')";
        try {
            $result = (int) $this->conn->fetchOne($query);
        } catch(Doctrine_Connection_Exception $e) {
            if ($onDemand && $e->getPortableCode() == Doctrine::ERR_NOSUCHTABLE) {

                try {
                    $result = $this->conn->export->createSequence($seqName);
                } catch(Doctrine_Exception $e) {
                    throw new Doctrine_Sequence_Exception('on demand sequence ' . $seqName . ' could not be created');
                }
                return $this->nextId($seqName, false);
+           } else {
+             throw new Doctrine_Sequence_Exception('sequence ' .$seqName . ' does not exist');
            }
        }
        return $result;
    }