Hello i´m trying to set info from a post method into a variable that sets a session named name the value that the user input. I get the following error:

Notice: Undefined index: name in F:\xampp\htdocs\Impossible game\index.php on line 18

This is line 18: $session = $_POST['name'];

<form action="ms1.php" method="POST">
Name <input type="text" name="name">
<input type="Submit" value="Begin">
</form>
<?php
$session = $_POST['name']; 
session_start();
$_SESSION['name'] = $session;