The armed robber entered the Caridean Street store about 7.50pm.
He selected an item before approaching the counter and wielding the knife at a staff member.
The offender demanded cash, but left empty-handed.
He was thought to be aged 18 to 20 and between 170cm and 175cm tall.
He had a skinny build and was wearing a grey hoodie.
Anyone with information can call Crime Stoppers on 1800 333 000.