Database - Query failed when we try to insert

I can't seem to get my Database to insert a customer...

It's in danish (the tags) but i think you can understand what happens anyways. We do connect to the database on the website, but it saids query failed...

Here's my contact form code:

<form action="elev.php" method="post">

        <fieldset>

        <label for="navn" style="float:left;">Navn:</label>

        <input type="text" id="navn" name="navn" placeholder="Andreas Andreasen" />



                    <label for="email">Email:</label>

        <input type="text" id="email" name="email" placeholder="Andreasandreasen@hotmail.com" />



                    <label for="telefonnummer">Telefon:</label>

        <input type="number" id="telefonnummer" name="telefonnummer" placeholder="12345678" />



                    <label for="postnummer">Postnr:</label>

        <input type="text" id="postnummer" name="postnummer" placeholder="8000" />



        <input style="float:right;" type="submit" value="Opret" name="submit" />



        </fieldset>

    </form>

    <form action="index.php">

            <input style="float:left; margin-top:-51px;" type="submit" value="Startsiden">

    </form>

Here's my PHP code for the database:(my password is not xxxxx, that's just to protect the database :) but it does say "connect succes! so i believe that is not the problem.

The problem comes when we try to put in a new customer and it saids query failed, the customerid = kundeid is incremental so we do not have that on the form.

    <?php

$mysqli=mysqli_connect("localhost", "f413f_gr6", "xxxxxx", "f413f_gr6");

if (mysqli_connect_errno()) {

    echo "Failed to connect: " . mysqli_connect_errno(); }

else {

    echo "Connect success!<br>"; }



    function performQuery($sql) {       

    global $mysqli;     

    if (mysqli_query($mysqli, $sql)) {          

    echo "Query Success!";      }

    else {          

    echo "Query Failed!";       }   

    }



    if (isset($_POST['submit'])) {



    $navn = $_POST['navn'];

    $email = $_POST['email'];

    $telefonnummer = $_POST['telefonnummer'];

    $postnummer = $_POST['postnummer'];



    performQuery("INSERT INTO kunder(navn, email, telefonnummer, postnummer) VALUES ('$navn', '$email', $telefonnummer, '$postnummer')");

    }

    else {

    echo "Tryk på submit"; 

     }        

?>

And here's a picture of our database:
http://imgur.com/Evezrtm