ajax callbacks not being called

ajax callbacks not being called

I'm having problems with callbacks in the code below:

$.ajax({
        dataType : 'jsonp',
        jsonpCallback : 'get_json',
        success : function(d) { alert(d); },
        error : function() { alert("ouch"); },
        url  : data_url
    })}

I have verified that the server is returning valid json.  My jsonpCallback is called, but neither the success nor the error callbacks are.  I'm stumped by this.