Parser error when using jQuery-UJS for remote link_to in Rails 3 app: how can I debug this?

I'm trying to replace the content of a div after clicking on a link using Rails 3, remote_link :remote => true and jQuery.

So far, I've been able to get the controller render the correct partial while responding with a 200 HTTP code. I've set some callbacks to find the origin of the problem:

jQuery(function($) {
    $("#follow-link").bind("ajax:before", function() {
        console.log("ajax:before");
    });

    $("#follow-link").bind("ajax:success", function(data, status, xhr) {
        console.log("ajax:success");
    });

    $("#follow-link").bind("ajax:complete", function() {
        console.log("ajax:complete");
    });

    $("#follow-link").bind("ajax:error", function(xhr, status, error) {
        console.log("ajax:error");
        console.log(error);
    });
});

While before and complete are triggered, success is not and error outputs "parsererror". The content I get when I inspect the response in Safari's developers tools is a simple string.

Why would it raise a parsererror? How can I get more information about what's causing this error?