RCP Icon service not working

Question

I have been adding icons to org.eclipse.ui.commandImages, but when I try to look them up the command image service can't find them. Best demonstrated with code:

ICommandImageService imgService = (ICommandImageService) PlatformUI.getWorkbench().getService(ICommandImageService.class);
ImageDescriptor imgDesc = imgs.getImageDescriptor("some.command.id");

imgDesc will always be null. The image entries have all the required fields filled in. Any ideas?