ScrollPane has type of "movieclip" in attached movieclip

Question

var spw:MovieClip = contentsLayer.attachMovie("ScrollPaneWrapper", "ScrollPaneWrapper123",
            contentsLayer.getNextHighestDepth());
var sp_:ScrollPane = spw.sp;

Here typeof(sp_) == "movieclip" and I can't set any content to it. I've tried exporting it for ActionScript, exporting the wrapper movieclip "ScrollPaneWrapper" and "Export in Frame 1" and all combinations of these options.

What's more weird is that I have another Flash project in which I can access the ScrollPane as expected and I can't tell any differences between the two projects.

Casting the spw.sp to ScrollPane results in null.