jQuery - How to PUT JSON via Ajax?

I am trying to put some JSON formatted data via Ajax with jQuery to a server. My code looks like this:

$.ajax({
    type: "PUT",
    url: myURL,
    contentType: "application/json",
    data: {"data": "mydata"}
});

But on the server-side, I receive a data=mydata string, instead of the expected JSON. Firebug tells me the same.

Where is the error?