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Denise can't exactly remember the number of stops she made driving an Uber passenger who needed to run a bunch of errands -- fast.

"He needed to go from A to B, then B to C, then C back to A again," says Denise, who doesn't want to use her last name. "He told me, 'Oh my God, I'm so glad you're doing this. You're making my day so much easier.'"

She drove him around Los Angeles for more than 90 minutes, yet traveled only 35 miles or so in that time. And since Uber primarily uses mileage to calculate its fares, the ride didn't cost much. The grateful passenger said he wanted to give her a big tip: $20.

He tapped the tip into the app -- and paused, "You gotta be kidding me," he told her. "It's saying it's over the limit."

Uber's app refused to let Denise's passenger tip $20 (or more), and he didn't have any cash on him. He ended up giving her the most the app would allow: $14.80.

Denise has been a full-time Uber driver for six years and never saw a tip limit before. But then, it's only been possible to tip Uber drivers from the app for the past six months.

Now playing: Watch this: Uber limits how much you can tip drivers

Drivers had asked the company for years to include tipping, but Uber insisted passengers appreciated the convenience of a tip-free ride. Finally, after months of scandals, executive shakeups and strained driver relations, Uber launched in-app tipping in June.

"You told us what you want and it's time we step up and give you the driving experience you deserve," the company wrote on its website at the time. "Because simply put, Uber wouldn't exist without you,"

Generosity should be something that you have no limit on. Denise, Uber driver

It was a move meant to showcase a new Uber, an Uber that appreciates its drivers. But the company didn't say anything about a ceiling for those tips. For some, the omission is a sign that Uber still doesn't "get" drivers. Others see it as classic tone deafness in a company that's working to move beyond lip service. No matter how you look at it, though, it's clear Uber needs to address this issue, which could push drivers to defect.

Uber limits

A glance through driver forums, blogs and social media groups shows that the tip limit has caught many Uber drivers by surprise. Dozens of drivers posted stories similar to Denise's, asking what gives.

"I figured it was just an early-on glitch. It seemed like a bunch of drivers kept emailing us about this," says Harry Campbell, who drives for both Uber and Lyft and runs the popular Rideshare Guy blog. "They never said anything about there being a limit."

Uber confirmed to CNET it does have a limit to safeguard against "fat fingers." You know the problem: You want to tip $10 but accidently type $100 or $1,000. This way, you won't have to go through the pain and hassle of getting your money back.

Uber's tipping limit is "200 percent of the total, up to $100," a company spokesman says. That lets a passenger, say, tip $50 on a $25 fare. "Of course, riders are free to tip additional amounts in cash if they'd like."

Dara Kerr/CNET

Campbell thinks other reasons may have factored into the tip limit, such as avoiding scams and the 3 percent fee Uber pays credit card companies. Uber declined to confirm this.

The ride-hailing service, founded in 2009, has experienced issues with scams in the past. For example, there have been instances when riders and drivers arrange fake trips, so that Uber pays the driver but the passenger has no intention of handing over the fare.

As for the incident with Denise, Uber says it shouldn't have happened. The $20 tip was well below that ride's 200 percent limit. The company says passengers and drivers should contact customer support if they run into that problem.

Tipping can be tricky

Lyft has offered in-app tipping for more than five years, but limits gratuities to $50 or 200 percent of the cost of the ride, whichever is lower. Like Uber, it says it aims to protect riders from fat-finger typos.

But even with that $50 limit, many drivers say they make better tips with Lyft than with Uber. Driver Will Preston even did some data crunching on hundreds of past rides and wrote a blog post for the Rideshare Guy about his findings in November. Turns out he made double the tips with Lyft.

One reason may be how each app is designed. With Lyft, the app displays a picture of your driver when you reach your destination and presents the option to tip. After that, you'll be taken to another screen where you can rate your driver.

Uber's app works a little differently.

Until a couple of months ago, tipping a driver involved reopening the app once you got out of the car and clicking on the question, "How was your trip?" You'd see the option to tip only after you rated your driver.

In other words, it was kind of a pain.

In November, Uber modified its app, which the company says accounts for 15 million rides worldwide per day. Now it says it sends riders a reminder to rate their drivers when they leave the car. But customers still have to go through that rating process before they see the tip option.

"Less than one-eighth of the customers ever rate a driver, and then never see the tipping option as it is hidden beneath the rating system that they rarely use," James Worley, a Los Angeles-based driver says. "They added it, sure, but do they really want the drivers to have it better or not?"

Uber's new CEO Dara Khosrowshahi says everyone should give big tips -- so they can get good ratings from drivers. "I am a very aggressive tipper right now. I pick the highest tip every time," Khosrowshahi told an audience at the World Economic Forum last week. "Everybody, tip aggressively."

But with a limit, that's not always possible.

"I don't think about it too often, because, you know, my blood pressure," Denise says. "Generosity should be something that you have no limit on."

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