Credit Vouchers

On 9/24/2006 due to overbooking my husband and I were given 2 vouchers for a free trip. The vouchers do not list an expiration date. I tried to use these vouchers this month and was told by NWA that they expire after one year. I looked on their internet sight under credit vouchers and it also does not say anything about an expiration date. If there had been an expiration date I would have used them before they expired.