Can't initialize an empty <ul> tabset

Both Firefox 3.0.5 and Opera 9.02 'hang' or break the flow of execution when attempting to initialize an empty <ul></ul> as a tabset.

If tabtest is a div containing an empty <ul></ul>, then this will fail :

$("#tabtest > ul").tabs();

$("#tabtest > ul").tabs("add", "#fragment1", "tab1");

But if there's something already in the <ul>, then it will succeed, like so :

$("#tabtest > ul").append("<li><a href='#fragment0'><span>tab0</span></a></li>");

$("#tabtest > ul").tabs();

$("#tabtest > ul").tabs("add", "#fragment1", "tab1");

In the attached .html, you should see an alert("done!") if and when the browser has finished creating the tabs. In the failure case, this alert never appears.