jQuery & Ajax – POST 406 error on form submit

I see the POST 406 error in developer console when I try to press the submit button on a contact form:

HTML:

<form id='contact-form' method="POST">
    <input name="name" type="text" class="text cleartext" placeholder="Your Name" />
    <input name="email" type="text" class="text cleartext" placeholder="Your EMail Address" />
    <textarea name="message" placeholder="Your Message"></textarea>
    <input type="submit" class="submit" value="Send" name="" />
</form>

Error:

jquery.1.9.1.min.js:5
POST https://example.co.uk/php-mail/mail.php 406

send @ jquery.1.9.1.min.js:5
ajax @ jquery.1.9.1.min.js:5
(anonymous) @ theme.mail.js:14
dispatch @ jquery.1.9.1.min.js:3
v.handle @ jquery.1.9.1.min.js:3

What could be causing this error?