Don’t be tempted by Pokemon Go lures in empty parking lots in the middle of the night.

With server troubles continuing to make the newly released smartphone game a slog to play during peak hours, a lot of people are heading out in the middle of the night to catch virtual creatures in real-life locations. Some clever and especially nerdy crooks have used that to their advantage, robbing folks they find alone at night hunting for new Pokémon.

The O’Fallon, Missouri Police Department posted an alert on its Facebook page about the crimes during the weekend, describing how robbers were using Pokémon Go to track down people who were out late at night and likely alone or in small groups.

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Pokémon Go works by encouraging you to get outside and walk around. The central premise of the game is that there are Pokémon out in the actual world, and using your smartphone, you can go catch them. The game is full of “PokéStops,” which are neighborhood landmarks like murals, eateries, churches and the like, which dispense useful items when you stop by. Players can also install limited-time “Lure Modules” into PokéStops, drawing Pokémon to a single location instead of having to walk around catching the little guys wherever they are. Lures are pretty fun in the game, in fact, because they can be used by anyone playing, not just the person who placed them, and often you’ll see lots of players who don’t know each other congregating at a lure to catch whatever Pokémon roll up.

Trouble is, at 2 a.m., when sleepless Pokémon trainers are on the prowl, dropping a lure at a Pokéstop is more like turning on a big flashing red light that says “I’m standing here alone, come rob me,” apparently. In O’Fallon, the crooks could see late-night lures in their Pokémon Go game—specifically the ones that happened to be in places like empty parking lots—and head to the real-world locations, where insomniac Pokéfreaks were waiting helplessly.

Good news is, the O’Fallon cops have arrested and charged three adult suspects and a juvenile for the robberies. So maybe the streets are safe for Pokémon trainers once again. Still, “don’t pop a lure in the middle of an empty parking lot at 2 a.m.” is pretty sound advice, whether you might get robbed or not. In fact, maybe just be careful about wandering around in the dark looking for fictional animals.