ORA-00936: missing expression หาไม่เจอว่าผิดตรงไหน ช่วยหาหน่อยครับ

//Oracle Class
#region Oracle
public DataSet OracleGet(string sql, string tblName)
{
	OracleConnection conn = new ConnectDB().OracleStrCon(BillConnStr);
	OracleDataAdapter da = new OracleDataAdapter(sql, conn);
	DataSet ds = new DataSet();
	da.Fill(ds, tblName);
	return ds;
}
public DataSet OracleGet(string sql, string tblName, OracleParameterCollection parameters)
{
	OracleConnection conn = new ConnectDB().OracleStrCon(BillConnStr);
	OracleDataAdapter da = new OracleDataAdapter(sql, conn);
	DataSet ds = new DataSet();
	foreach (OracleParameter param in parameters)
	{
		da.SelectCommand.Parameters.Add(param.ParameterName, param.OracleDbType).Value = param.Value;
	}
	da.Fill(ds, tblName);
	return ds;
}
public int OracleExecute(string sql)
{
	int i;
	OracleConnection conn = new ConnectDB().OracleStrCon(BillConnStr);
	OracleCommand cmd = new OracleCommand(sql, conn);
	conn.Open();
	i = cmd.ExecuteNonQuery();
	conn.Close();
	return i;
}
public int OracleExcute(string sql, OracleParameterCollection parameters)
{
	int i;
	OracleConnection conn = new ConnectDB().OracleStrCon(BillConnStr);
	OracleCommand cmd = new OracleCommand(sql, conn);
	foreach (OracleParameter param in parameters)
	{
		cmd.Parameters.Add(param.ParameterName, param.OracleDbType).Value = param.Value;
	}
	conn.Open();
	i = cmd.ExecuteNonQuery();
	conn.Close();
	return i;
}
#endregion

for (int r = 0; r < dtUNIT_MASTER_ToBill.Rows.Count; r++)
{
	sql = "insert into UNIT_MASTER (USER_SEQ,BILL_PERIOD,USERNAME,USERADDR,USERTYPE,METER_TYPE,CALC_CODE,PRESSURE_CODE,SERIAL_NO,PEA_NO,INSTALL_DATE,PHASE_CODE,MULTI_UNIT,CURR_UNIT1) ";
	sql += "values(@user_seq,@bill_period,@username,@useraddress,@usertype,@meter_type,@calc_code,@pressure_code,@serialnumber,@peanumber,to_date(@install_date,'DD/MM/YYYY HH24:MI:SS'),@phase_code,@multi_unit,@curr_unit1); "; 
	OracleParameterCollection param = new OracleCommand().Parameters;
	param.Add("user_seq", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["USER_SEQ"].ToString();
	param.Add("bill_period", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["BILL_PERIOD"].ToString();
	param.Add("username", OracleDbType.Varchar2).Value = dtUNIT_MASTER_ToBill.Rows[r]["USERNAME"].ToString();
	param.Add("useraddress", OracleDbType.Varchar2).Value = dtUNIT_MASTER_ToBill.Rows[r]["USERADDR"].ToString();
	param.Add("usertype", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["USERTYPE"].ToString();
	param.Add("meter_type", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["METER_TYPE"].ToString();
	param.Add("calc_code", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["CALC_CODE"].ToString();
	param.Add("pressure_code", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["PRESSURE_CODE"].ToString();
	param.Add("serialnumber", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["SERIAL_NO"].ToString();
	param.Add("peanumber", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["PEA_NO"].ToString();
	param.Add("install_date", OracleDbType.Date).Value = Convert.ToDateTime(dtUNIT_MASTER_ToBill.Rows[r]["INSTALL_DATE"]);
	param.Add("phase_code", OracleDbType.Char).Value = dtUNIT_MASTER_ToBill.Rows[r]["PHASE_CODE"].ToString();
	param.Add("multi_unit", OracleDbType.Double).Value = Convert.ToDouble(dtUNIT_MASTER_ToBill.Rows[r]["MULTI_UNIT"]);
	param.Add("curr_unit1", OracleDbType.Double).Value = Convert.ToDouble(dtUNIT_MASTER_ToBill.Rows[r]["CURR_UNIT1"]);
	int i = new DBClass().OracleExcute(sql, param);
}