mysqli_num_rows()

alguém me ajuda, Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given in listarempregos.php on line 9

<?php
if(@$_GET["acao"] == "lista"):
?>

<?php
$sql = "SELECT * FROM empregos ORDER BY nome ASC";
$sql2 = mysqli_query($connect, $sql);

if(mysqli_num_rows($sql2) == false){

	echo '<br /><div align="center">Nenhuma anuncio cadastrado.</div>';
	exit;
}
?>
<br />
<table border="0" cellpadding="0" id="table" >
 <tr style="background-color:#272727; height:30px; width:auto; color:#B9B9B9;">
 <td width="12%" height="26" align="left" valign="middle"> </td>
 <td width="51%" height="26" align="left" valign="middle"> Nome</td>
 <td width="12%" align="center" valign="middle">Status</td>
 <td width="11%" align="center" valign="middle"> </td>
 <td width="14%" align="center" valign="middle"> </td>
 </tr>
<?php

while($ln = mysqli_fetch_object($sql2)): ?>