Some windows on your screen are managed by X11 but the content is not. Under those circumstances, the content is not defined to X11 and X11 returns gibberish when asked to return the pixels by ImageMagick. An example might be a window managed by X11 buts its content defined by GL.
Try adding -screen and see if that makes a difference. If not, you may need to use another program that
reads the graphics buffer directly rather than relying on X11 to define the pixels.