ORA-01036: 非法的变量名/编号

public bool ModifyDrawBill(MDrawBill mBill)
        {
            StringBuilder strSQLText = new StringBuilder();
            strSQLText.Append("update DrawBill set ");
            OracleParameter[] param = 
            {
new OracleParameter("PKID", OracleType.Int32),
new OracleParameter("StationId", OracleType.VarChar),
new OracleParameter("BillBegin", OracleType.VarChar),
new OracleParameter("BillEnd", OracleType.VarChar),
new OracleParameter("SumCount", OracleType.Int32),
new OracleParameter("DrawDate", OracleType.DateTime),
new OracleParameter("IsConfirm", OracleType.Int32),
new OracleParameter("AppUser", OracleType.NVarChar),
new OracleParameter("CheckUser", OracleType.NVarChar),
new OracleParameter("Memo", OracleType.NVarChar)
            };
            //@@lwd 20110803
            param[0].Value = mBill.PKID;
            if (mBill.StationId != null)
            {
                strSQLText.Append("StationId=:StationId,");
                param[1].Value = mBill.StationId;                
            }
            if (mBill.BillBegin != null)
            {
                strSQLText.Append("BillBegin=:BillBegin,");
                param[2].Value = mBill.BillBegin;
            }
            if (mBill.BillEnd != null)
            {
                strSQLText.Append("BillEnd=:BillEnd,");
                param[3].Value = mBill.BillEnd;
            }
            if (mBill.SumCount != null)
            {
                strSQLText.Append("SumCount=:SumCount,");
                param[4].Value = mBill.SumCount;
            }
            if (mBill.DrawDate != null)
            {
                strSQLText.Append("DrawDate=:DrawDate,");
                param[5].Value = mBill.DrawDate;             
            }
            if (mBill.IsConfirm != null)
            {
                strSQLText.Append("IsConfirm=:IsConfirm,");
                param[6].Value = mBill.IsConfirm;
            }
            if (mBill.AppUser != null)
            {
                strSQLText.Append("AppUser=:AppUser,");
                param[7].Value = mBill.AppUser;
            }
            if (mBill.CheckUser != null)
            {
                strSQLText.Append("CheckUser=:CheckUser,");
                param[8].Value = mBill.CheckUser;
            }
            if (mBill.Memo != null)
            {
                strSQLText.Append("Memo=:Memo,");
                param[9].Value = mBill.Memo;
            }
            if (strSQLText.ToString().EndsWith(","))
            {
                strSQLText = strSQLText.Remove(strSQLText.ToString().Length - 1, 1);            }
            strSQLText.Append(" where PKID=:PKID ");
            return OracleHelper.ExecuteNonQuery(OracleHelper.Connection_String, CommandType.Text, strSQLText.ToString(), param) > 0;//提示“RA-01036: 非法的变量名/编号”        }