membership access提供程序更改密码错误问题？

这是我用的membership提供程序里的的一个方法，为什么改密码时一直报错，我用vs设了断点
说是update的语句有错误，大家帮我看看，谢谢！
OleDbConnection conn = new OleDbConnection(connectionString);
            OleDbCommand cmd = new OleDbCommand("UPDATE [" + tableName + "]" +
                    " SET Password = ?, LastPasswordChangedDate = ? " +
                    " WHERE Username = ? AND ApplicationName = ?", conn);

            cmd.Parameters.Add("@Password", OleDbType.VarChar, 255).Value = EncodePassword(newPwd);
            cmd.Parameters.Add("@LastPasswordChangedDate", OleDbType.DBDate).Value = DateTime.Now;
            cmd.Parameters.Add("@Username", OleDbType.VarChar, 255).Value = username;
            cmd.Parameters.Add("@ApplicationName", OleDbType.VarChar, 255).Value = pApplicationName;

            int rowsAffected = 0;

            try
            {
                conn.Open();

                rowsAffected = cmd.ExecuteNonQuery();  //调试时说错误出在这里
            }
            catch (OleDbException e)
            {
                if (WriteExceptionsToEventLog)
                {
                    WriteToEventLog(e, "ChangePassword");

                    throw new ProviderException(exceptionMessage + "1");
                }
                else
                {
                    throw e;
                }
            }