Index was out of range c#

Why i get index was out of range? I watched my code many time, and I cannot find where is mistake. Here is list:

 List<PictureBox> bullets = new List<PictureBox>();
 int i=0;

Event KeyDown:

void Tank_KeyDown(object sender, KeyEventArgs e)
{

        if (e.KeyData== Keys.space)
            {
            if (bullets.Count==0)
                {
                    PictureBox bullet = new PictureBox();
                    kulka.Name = Convert.ToString(i);
                    kulka.Image = new Bitmap("foto/kulka2.png");
                    Random dd = new Random();
                    if (pagr.Name == "RotateW")
                        bullet.Location = new Point(pagr.Location.X + 16, pagr.Location.Y - 8);
                    if (pagr.Name == "RotateS")
                        bullet.Location = new Point(pagr.Location.X + 16, pagr.Location.Y + pagr.Height + 2);
                    if (pagr.Name == "RotateA")
                        bullet.Location = new Point(pagr.Location.X - 8, pagr.Location.Y + 16);
                    if (pagr.Name == "RotateD")
                        bullet.Location = new Point(pagr.Location.X + pagr.Height + 11, pagr.Location.Y + 16);
                    bullet.Width = 5;
                    bullet.Height = 8;
                    bullet.SizeMode = PictureBoxSizeMode.StretchImage;
                    bullet.BringToFront();
                    bullets.Add(bullet);
                    panelis.Controls.Add(bullets[i]);
                    Task task = new Task(() => this.bullet_move(i, pagr.Name));
                    task.Start();
                    this.Refresh();
                }
            }
}

And here is my task:

void bullet_move(int j,string rotation)
    {
        while (true)
        {
                    if (rotation == "RotateW")
                    {
                            BeginInvoke(new Action(() => bullets[j].Top = bullets[j].Top - 1));

                        if (bullets[j].Top < 2)
                        {
                            BeginInvoke(new Action(() => panelis.Controls.Remove(bullets[j])));
                            bullets.RemoveAt(j);
                            Application.DoEvents();
                            i = 0;
                            break;
                        }
                    }
           } 
      }

Sometimes I get here index out of range:

BeginInvoke(new Action(() => bullets[j].Top = bullets[j].Top - 1));

and sometimes here:

  BeginInvoke(new Action(() => panelis.Controls.Remove(bullets[j])));

Why I get that error? :/ Thanks for help... :)