php mysql_fetch_array() 에러

$bal1=mysql_query("select balance, acc_type from customer where custid=$_SESSION[username]"); // line 66
while($b = mysql_fetch_array($bal1, MYSQL_BOTH)) //line 67
{
$bal=$b[0];
$acc=$b[1];
}
 
$min1=mysql_query("select minbalance from accounts where acc_type='$acc'");
while($a = mysql_fetch_array($min1, MYSQL_BOTH))
{
$min=$a[0];
 
}
 
$available=$bal-$min;