Rotating Bitmap

Ok, I’m getting a weird error on something that should be way too easy. It’s only a few lines of code so I’m hoping I’m just missing something stupid. Hopefully someone else can see the error.

// I'm using the BulkLoader class to import my images. 
// I'm creating a new Bitmap because I'm reusing the same image lots of times.
var Grass31:Bitmap = new Bitmap(BulkLoader.getLoader("main-site").getBitmapData("Grass"));
Grass31.rotation = 180;
// Adding it to a larger clip so that I can move and manipulate the whole thing.
var rd31:MovieClip = new MovieClip();

When I execute, I get:
ArgumentError: Error #2015: Invalid BitmapData.
at flash.display::BitmapData()

Seriously. WTF? I’ve done this a million times. I gotta be missing something small.

Thanks for your help!