errinho q to apanhando (gerenciador d e-mails)

o erro q da é

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /home/eshopbr/public_html/MyML/envio.php on line 33

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31- $sql_1 = "SELECT * FROM users";

32- $res_1 = mysql_query($sql_1);

33- $total = mysql_num_rows($res_1);

34- $total_2 = ($total + 1);

35-

36- if ($id > "0" AND $id < "$total_2"){

37-