If you’re the type of person who enjoys eating out at restaurants, you may lose your appetite after reading about a new “financial etiquette” survey from TD Ameritrade.

With help from the Harris Poll, the online broker asked 1,011 U.S. adults which types of workers they’re most likely to tip, and the results are a bit unsettling. Just 82% of respondents said they typically leave a tip for waitstaff at restaurants. That means 18% of diners not only think it’s okay to stiff those hardworking tipped-wage workers, but they’re comfortable enough in their belief to say so in a survey.

The numbers are even more stark when broken down by age group: 91% of baby boomers said they tip restaurant staff, while only 81% of Gen Xers and 72% of millennials said the same.

Just so there’s no confusion, those numbers should be 100%, 100%, and 100%. Restaurant workers, by and large, rely on tips as part of their wages. While there are honest debates to be had about the fairness of tipping as a whole, as long as the tipping system is in place, fair-minded patrons can’t just opt out of it. Not tipping a waiter or waitress doesn’t upend the system—it only penalizes workers who have no control over it.

The survey showed that other types of workers are getting stiffed even more frequently. Only 61% of respondents said they typically leave a tip for bartenders, for example. (How these people ever get a second drink at the same bar is beyond me.) Meanwhile, just 45% said they tip taxi drivers and 35% said they tip hotel workers.

As for those tip jars you often see strategically placed at fast-food joints and coffee shops? Just 26% of respondents said they toss in their cash.