我用combox的SelectedIndexChanged事件出现错误

运行程序，出现InvalidArgument=“0”的值对于“SelectedIndex”无效。        private void cmbBmkt_SelectedIndexChanged(object sender, EventArgs e)
        {
            
            if (cmbBmkt.SelectedIndex != -1 || cmbEmkt.SelectedIndex != -1)
            {
                if (cmbEmkt.SelectedIndex < cmbBmkt.SelectedIndex)
                {
                    cmbEmkt.SelectedIndex = cmbBmkt.SelectedIndex;
                }
            }
        }