Problem With OR Inside A Trigger

I created the following trigger to ensure that Mastercard or Visa are entered as the credit card type is a credit card table. The trigger works fine when I use one type, but when I add or, it doesn't work. Any advice? Thanks



/* THIS TRIGGERS VERIFIES THAT A VALID CREDIT CARD TYPE IS ENTERED*/

CREATE TRIGGER trg_accepted_credit_card
on credit_card_payments
for insert
as
begin

declare @card_type varchar(50)

select @card_type = (select card_type from inserted)

if exists(select 1 from inserted where @card_type != 'Mastercard' OR @card_type != 'Visa')

begin
print 'Invalid Credit Card Type.'
rollback transaction
end
end