NullPointerException in JCodec with .mkv

I want to get all frames from video in jpg. For .mp4 file program works great but for .mkv i got NullPointerException I don't know why. I use JCodec

public static void Frames(File video, String destiny){
    boolean check = true;
    int i = 1;

    while (check == true) {
        try {
            BufferedImage frame = FrameGrab.getFrame(video, i);  //22
            ImageIO.write(frame, "jpg", new File(destiny+"/photo" + i + ".jpg"));
            System.out.println("Now: " + i);
        } catch (ArrayIndexOutOfBoundsException e) {
            check = false;
        }
        catch(JCodecException e){
            e.printStackTrace();
        }
        catch(IOException e){
            e.printStackTrace();
        }
        i++;
    }
}

.

Exception in thread "main" java.lang.NullPointerException
at org.jcodec.api.FrameGrab.<init>(FrameGrab.java:59)
at org.jcodec.api.FrameGrab.getFrame(FrameGrab.java:331)
at Pack.Main.Frames(Main.java:22)
at Pack.Main.main(Main.java:77)

Thanks