jQuery UI AutoComplete source option - function callback

Just a quick one: if you have a jQuery UI AutoComplete and you want to use the source option to set a function that does a remote lookup, you'll be aware that the function signature is as follows:

function(request, response)

where request is what the user entered, and response is a function reference that accepts an array of results to display.

The prob we had today was that we wanted to call the response function from the success of a $.ajax() call. But it wouldn't work, and it took ages to figure out that this was because the call was in asynchronous mode. Once we set the $.ajax() async option to false, it worked fine.