Two men entered the premises on Scrooby Road just before 8.30pm and demanded money. They fled with a quantity of cash.
Police said there were no weapons seen and no-one was injured during the robbery.
No customers were in the shop at the time.
The men, who covered their faces, were white and wearing dark coloured hooded tops.
Police believe they joined a third man in the car park before leaving the scene on foot.
Detective Inspector Ash Wilson said: “This type of incident is uncommon and, although the staff member was incredibly shaken by the ordeal, thankfully no one was injured.”
“Patrols have been increased in the area to provide reassurance.”
“If you have any information about those responsible please call us.”
If you saw the men, just before the robbery or in the moments afterwards, contact police on 101, quoting incident 773 of 27 January 2014. Alternatively call Crimestoppers anonymously on 0800 555 111.