The startup has run out of funding, according to an email obtained by Fortune that the company sent to customers.

“We made enormous progress – that you’ve witnessed firsthand – and edged toward profitability,” the email said. “Unfortunately, however, our funding has prevented us from continuing on our quest toward building out our vision and delivering on our long term goals.”

Four-year-old RidePal provided luxury commuter shuttle rides for corporate customers and individuals in the Bay Area and Los Angeles. It charged between $4.75 and $12 per ride.

As it hints in the email, transportation businesses can be tricky to make work financially, and the company likely couldn’t sustain itself with the current model.

RidePal has raised $3.7 million in total funding from investors like Claremont Creek Ventures and Volvo Group Venture Capital. Fellow commuter bus startup Chariot, which has been operating in San Francisco for less than a year, has already raised $3 million. Meanwhile, Leap Transit, has raised $100,000 and is currently on its second hiatus as it deals with operational and regulatory issues in San Francisco.

We’ve reached out the company for comment and will update if we hear back.