缺少所需数据时显示错误404

I have a page that depends on MySQL Data. If a row its looking for does not exist, I want to show an error 404.

I already did something like this

Note: $data = is the query result

$count = mysqli_num_rows($data);

if($count==0) {
    header("HTTP/1.0 404 Not Found");
} else {
    //Do Nothing
}

The $count does return a 0 value.

Nothing's Happening :C Help pls