ActionScript 3.0 :: LoaderInfo Url As Null For Too Long

I have a class that loads a handful of external swfs, and a preloader class to handle them all as a single object.I was having some strange problems with the loading process and tried tracing the urls to see which file was bugging. For my surprise, almost all loaderInfo objects reported theirs url as null.With a bit of testing I came to the conclusion that they only report some url when the file is almost completely loaded. The docs say something about "possibly wrong urls" before progress events, but that would be in cases where redirect happen, and everything here is local with simulate download option. Also, "event.target.url" inside the very progress event handler reports null unless the target's loading is almost done. Unfortunately I don't have any piece of code with less than a hundred lines to show. But is there any undocumented behavior, known bug or common coding error that generates such situation?