c# an object on upper level cannot be added to an object

private void frmInitialEntryBooks_Load(object sender, EventArgs e)
{
    frmSujbect objForm = new frmSujbect();
    objForm.TopLevel = false;
    pnl.Controls.Add(objForm);
    objForm.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
    objForm.Dock = DockStyle.Fill;
    objForm.Show();
}