Call to a member function query() on a non-object in c:\\ …..

<?php 

if(isset($_REQUEST['catid']))
  {
     global $wpdb;
	$cat_id = $_REQUEST['catid'];
	$table_name = $_REQUEST['tablename'];
	$sql1 = "DELETE * FROM " . $table_name . " WHERE id=" . $cat_id;
	$wpdb->query($sql1);
	echo "<p class=\"deleted\"> Header image for this category ID " .$cat_id. " from ". $table_name." is Deleted</p>";
  }
?>

Call to a member function query() on a non-object in C:\wamp\www\wordpress\wp-content\plugins\dynamic header images\delete.php on line 14

anyone please help me out

Thank you
khiangte