actionscript: DownloadProgressBar and Flex 4 can't find resource bundle

I am using flex 4 and I want to custom the preloader, but I meet error of DownloadProgressBar. The code:

    public class MyPreLoader extends DownloadProgressBar
{
    var progress:ProgressBar = new ProgressBar();

Or substitude ProgressBar to Label will produce the same error:

Error: Could not f开发者_Python百科ind compiled resource bundle 'collections' for locale 'en_US'.

Anyone has implemented the custom preloader? I am almost missing here.

Resolved by adding:

[ResourceBundle("")] tag in the source!

Hope this can help others.