AAAAGGG MySQL error?

For some reason, /I get an error when this php code is run? I cant figure out why
if ((isset($activity_id)) && (isset($company_id))) {
$sql4="SELECT c_name from company WHERE c_id = $_GET[company_id]";
$query4=mysql_query($sql4);
$result4 = mysql_fetch_array($query4);
$sql1="SELECT next, medivas, a_desc, date_format(a_date,'%d/%m/%Y') AS a_date_formatted from activity WHERE a_id = $_GET[activity_id]";
$query1=mysql_query($sql1);
$result1 = mysql_fetch_array($query1);
}
heres the error I get (Line 27 is the 2nd to last line.)

Warning: Supplied argument is not a valid MySQL result resource in /home/domains/lukeu/lukesplace.com/Padilla/deleteactivity.php on line 27