Add error parameter to Google reCaptcha

I'm having some trouble implementing reCapthca into an asp.net web form. I've read through the docs provided by Google but when I try to add the error parameter to the challenge url the reCaptcha doesn't do anything at all and just lets the user right through to my success page. If I don't add the error parameter the reCaptcha functions properly but I can't seem to get an error message to display:
This is what I have in my webform:
<script type="text/javascript"src="https://www.google.com/recaptcha/api/challenge?k=my_public_key&error="> </script> <noscript> <iframe src="https://www.google.com/recaptcha/api/noscript?k=my_public_key" height="300" width="500" frameborder="0"></iframe><br> <textarea name="recaptcha_challenge_field" rows="3" cols="40"></textarea> <input type="hidden" nam ...