错误：ORA-12704：字符集不匹配

Question

SELECT
     null AS m_inout_id,
     'OPEN' AS documentno         
FROM
     adempiere.transaction_view trv    
UNION ALL
SELECT
     trv.m_inout_id AS m_inout_id,
     trv.documentno AS documentno
FROM
     adempiere.transaction_view trv

问题在于documentno。 documentno的数据类型是NVARCHAR2（20）。