This can be fixed by changing the isearch_continue_noninteractively function in modules/isearch.js to this:

function isearch_continue_noninteractively (window, direction) {
    var s = new isearch_session(window.minibuffer, direction);
    if (window.isearch_last_search) {
        s.find(window.isearch_last_search, direction, s.top.point);
        window.minibuffer.push_state(s);
        s.restore_state();
        isearch_continue (window, direction);
    }
    else
        throw "No previous isearch";
    isearch_done(window, true);
}

I'll gladly submit a patch or a pull request if you tell me where and how.

Thanks for a wonderful browsing experience :)