PHP if(isset) not working properly with 2 radio groups and 1 textname

I have an if statement that I need to return as both radio groups have been selected and the text box has been filled in or else, deliver and error message and for some reason it's not working, could someone help me figure out what I am missing? The 'agegroup' and 'music' are the radio groups and the 'firstname' is the text box.

<?php

if(isset($_POST['agegroup']) && isset($_POST['music'])
&&!($_POST['firstname'] == NULL))
{
?>
<h1>Thank For Taking the Survey <?php echo $_POST['firstname'] ?> </h1>
else
{
echo "<h1>Please return to the form and fill out completely</h1>";
}