insert null value error

[stan1334]

sqlite3
win10
64

//创建数据表sql
#define CREATE_HE_DATA_TABLE_SQL ("create table HE_DATA ("
"SOURCENODE varchar(256),"\

struct TestData{
std::string PARENTNODE;
int another;
XXX
}

    static void to_base(const TestData& p, values& v, indicator& ind)
    {
      v.set("PARENTNODE", p.parentNode, p.parentNode.empty() ? i_null : i_ok);

}

when i insert struct a field is null，soci err info::
what:Null value not allowed for this type while executing "insert into

soci::transaction tr(*sql);
T insertData;
soci::statement st = (sql->prepare << strSql, SQL_USE(insertData));

for (auto& data : datas) {
insertData = data;
st.execute(true);
errroIndex++;
}
tr.commit();

soci err info::
what:Null value not allowed for this type while executing "
insert into HE_DATA (EVENTID,SOUR XXX PARENTNODE=NULL,

create field default can write null
"SOURCENODE varchar(256),"\

or
"SOURCENODE varchar(256) not null,"
always have same err

Please help guide
@vadz

[stan1334]

Both get and set need to determine i_null