re-using a BitmapImage in multiple ImageBrushes results in "Value does not fall within the expected range"

I'm bumping into something I find rather odd -

I've got two XAML objects. Both are added dynamically as a child to my main canvas. I'm using a BitmapImage to set the background like so:
ImageBrush imgBrush = new ImageBrush();
BitmapImage bi = bitMaps[image];
imgBrush.ImageSource = bi;
blockXAML.canvasRoot.Background = imgBrush;

bitMaps is a static dictionary<string, BitmapImage> storing a BitmapImage. The very first time I reference it, it works great. But for the second xaml that uses it, I get an error saying " ...