Brooklyn — A black-garbed gunman went into a Brooklyn store where a woman was working alone yesterday, and when he found he couldn’t rob her, angrily dragged her into a back room and sexually assaulted her, police said. The thug, clad in black pants, black T-shirt and what his victim described as a black hat similar to one worn by the late singer Marvin Gaye, entered the furniture store in Bay Ridge at around 1 p.m., cops said.

“I haven’t got any money,” she told him when he pulled a gun and announced a holdup, police sources said.

There were no customers in the store at the time and he took her to a back room and forced her to perform oral sex on him, cops said.

The Post is not naming the store or giving its exact location to protect the woman’s identity.

The gunman, described as black, 5-feet-11 tall and of medium build, fled.

His victim was taken to an area hospital for observation, cops said.

The owner of a beauty salon nearby said that “usually it’s quiet around here. This is the first time I’ve heard of something like this.”

City Council member Vincent Gentile went to the scene and cautioned that “crime is down, but we have to stay vigilant.”

People in the area said there are usually male employees in the store and they were surprised to hear the woman was working solo yesterday.