Ida Modliba, 22, told The Local that she laughed when she first spotted the unsual no-smoking sign after she and her boyfriend checked in at the Castle House Inn in Stockholm's Old Town (Gamla Stan).
But first, we ask sensitive readers to look away now.
The sign, put up in one of the communal toilets, warned hostel guests in no uncertain terms: "No smoking or wanking. It is a criminal offence to smoke or wank on these premises."
It was accompanied by a crossed-out cigarette (just a cigarette).
Okay, fine, here's a picture:
A picture of the sign on the door. Photo: Ida Modliba
"I started googling to check if 'wanking' could mean different things in different situations. But it looks like it means only one thing," Modliba told Expressen after it first wrote about it.
For those of our readers who may not be native English speakers, wanking is chiefly British, and rather vulgar, slang for masturbating.
But if you're dying to bop the baloney, don't worry. It looks like the sign is meant as a joke.
"We have put up several different signs with various informative messages. I thought that one was funny," hostel manager Henrik Thydén told Expressen.
Not everyone saw the humorous side, however. One guest commented on Booking.com: "I found the sticker on the door offensive and disrespectful for the guests."