ORA-01084: OCI 调用中的参数无效

clob,nclob当值为“”空字符串时，就会出现这个错误

    OracleParameter op = new OracleParameter("CONTENTclog", OracleType.Clob);
            if (model.CONTENT == ""||model.CONTENT==null)
            {
                op.Value = " ";
            }
            else
            {
                op.Value = model.CONTENT;

            }
            OracleParameter[] o = { op };