✅ Cant convert a list to list of a class type?

public partial class frmCustomers : Form
    {
        List<Customer> customers = new List<Customer>();

        private void btnAdd_Click(object sender, EventArgs e)
        {
            frmAddCustomer addCustomerForm = new frmAddCustomer();
            Customer customer = addCustomerForm.GetNewCustomer();
            if (customer != null)
            {
                CustomerDB.SaveCustomers(customer); /// This is the line giving me an error
                MessageBox.Show(customer.GetDisplayText().ToString(), "BTNAddClick2");
                CustomerListBox();
                
            }
        }