Undefined index: slide_title

function add_slides() {
if(isset($_POST['add_slide'])) {
$slide_title     = escape_string($_POST['slide_title']);
$slide_image     = escape_string($_FILES['file']['name']);
$slide_image_loc = escape_string($_FILES['file']['tmp_name']);

if(empty($slide_title) || empty($slide_image)) {

echo "<p class='bg-danger'> This field cannot be empty</p>";
}else {

move_uploaded_file($slide_image_loc, UPLOAD_DIRECTORY . DS . $slide_image);

$query = query("INSERT INTO slides(slide_title, slide_image) VALUES('{$slide_title}', '{$slide_image}')");
confirm($query);
set_message("slide added");
redirect("index.php?slides");
}	
}
}