EmptyResultDataAccessException

报此错误 :org.springframework.dao.EmptyResultDataAccessException: Incorrect result size: expected 1, actual 0,

查询的结果集 为null,导致报错 

@Transactional(readOnly = true)
public Map<String, Object> findActivityTask(final Long productId){
    return super.findBySQL(new SQLGenerator() {
        @Override
        public Map<String, Object[]> generate() {

            Map<String, Object[]> result = Maps.newHashMap();
            List<Object> list = new ArrayList<Object>();

            StringBuffer sql = new StringBuffer();
            sql.append(" SELECT d.id, d.criteria_tagline AS criteriaTagline FROM ex_product_base AS a ");
            sql.append(" LEFT JOIN ex_activity_gift AS b ON a.id = b.product_id AND b.state = 0 ");
            sql.append(" LEFT JOIN ex_activity_base AS c ON b.id = c.id AND c.state = 0 AND c.open_type = 2 ");
            sql.append(" INNER JOIN ex_activity_task AS d ON c.id = d.activity_base_id AND d.state = 0 AND d.open_type = 2 WHERE 1=1 ");

            if (productId != null) {
                sql.append(" AND a.id = ? ");
                list.add(productId);
            }
            sql.append(" ORDER BY c.create_date DESC LIMIT 1 ");

            result.put(sql.toString(), list.toArray());
            return result;
        }
    });
}