jQuery AJAX success handler not functioning as expected

I'm experiencing a very strange problem with jQuery and IE. I'm using jQuery 1.4.4 with IE8. I try an AJAX call with:

$.ajax({
        url: '/events/do_something',
        dataType: 'json',
 开发者_如何转开发       data : params,
        beforeSend : function() {
            setPageStateLoading(true);
        },
        success : function(data){
            doSomethingElse(data, false);
        },
        error : function(XMLHttpRequest) {
            openErrorDialog(XMLHttpRequest.responseText);
        },
        complete : function(XMLHttpRequest, textStatus) {
            setPageStateLoading(false);
        }
    });

Although the request is successful the error handler gets called because of a parse error. There's nothing wrong with the returned JSON (it works perfectly on Firefox), the problem is line 6229 in jQuery:

var ct = xhr.getResponseHeader("content-type") || "",

Which fails with "Object doesn't support this property or method" and causes a parse error.

Right now I'm using the complete handler with an if statement as a workaround but this isn't really a very good idea. Any ideas on how to fix this properly?

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Have you tried leaving dataType unset and then using jQuery's .parseJSON() (http://api.jquery.com/jQuery.parseJSON/)? This way you'll be able to see if there's an error in the response.