my SQLSetPos() always returnes -1.

Hi ALL.

my SQLSetPos() always returnes -1.
Here is my code.
sr=SQLFetch(m_MailSet.m_hstmt); //sr=0
sprintf((char*)myrows[0].Person,"xx%dxx",4);
sr=SQLSetPos(m_MailSet.m_hstmt,1,SQL_UPDATE,SQL_LOCK_NO_CHANGE); //sr=-1
try
{
m_MailSet.CheckRowsetError(sr);
}
catch(CDBException *e)
{
}

My table is a *.dbf table.
m_MailSet is a object of CRecordSet.
SQLBindCol() is OK.
The state got from e is s1010(HY010).

Any help will be appreciated.