A man claiming to have a gun in his jacket pocket robbed the Presto 31 Convenience Store shortly after midnight Wednesday.
The man entered the store, 1802 W. 23rd St., while the clerk was in a back room.
The clerk apparently forgot to lock one of the front doors, which made it possible for the man to enter the store, said Lawrence Police Department spokesman David Anderson.
Hearing someone enter, the clerk walked from the back room into the store. There he was confronted by a slender black man wearing a green, hooded jacket.
It appeared the man was holding a gun in the pocket of his jacket, Anderson said. The clerk never saw the gun.
The robber ordered the clerk to walk to the front of the store and open the cash register.
The man made off with an undisclosed amount of cash.
The robber was described as about 6 feet tall and weighing about 150 pounds.