Police were called to reports of a robbery at the premises in Queen Street shortly before 6.30am.
After threatening the woman and pocketing their haul, the robbers left the area in a blue car.
The woman was left shaken but avoided injury.
A spokesperson for Humberside Police said: “Officers are currently in attendance and a cordon is in place whilst initial investigations take place.”
Anyone with any information is asked to call the force via 101 quoting log 58 of November 28.