jQuery.getJSON doesn't trigger callback

I have a html code:

<button>asd</button>
<script type = "text/javascript">
$('button').click(
    function() {
        $.getJSON('/schedule/test/', function(json) {
            alert('json: ' + json + ' ...');
        });
    }
);
</script>

and corresponding view:

def test(request):
    if request.method == 'GET':
        json = simplejson.dumps('hello world!')
        return HttpResponse(json, mimetype = 'application/json')

The view is executed (tested using print), json variable is initialised but no alert appears. What did I do wrong? I've already seen some docs on this (http://docs.jquery.com/Ajax/jQuery.getJSON#urldatacallback for example) but I didn't find an answer.

EDIT: The problem was, that HttpResponse was not imported... Unfortunately Django gave no error about it. Everything else was correct. regards
chriss