Tesla Motors is suing German auto parts maker Hoerbiger Automotive Comfort Systems after the supplier allegedly failed to deliver the electric car maker working doors for its new Model X car, reports the Wall Street Journal.

The Model X’s swooping “falcon-wing” doors are one of its signature features, but were notoriously difficult to produce. At one point last year, Tesla TSLA CEO Elon Musk called the Model X “the hardest car to build in the world.”

Partly because of the doors and other engineering decisions, the Model X was delayed by many months. The company first showed off a prototype of the car in early 2012, and only started delivering the first cars to customers in September 2015.

Photograph by Bloomberg via Getty Images

The lawsuit gives a glimpse into just how much of a pain those doors were to get right.

Tesla accuses Hoerbiger of misrepresenting that it could deliver functioning doors, which ended up forcing Tesla to switch to a new supplier in the spring of 2015. That was only a few months before it was due to ship the first cars to customers. Because of the 11th hour change, Tesla had to pay its new supplier a premium for the doors.

Tesla made just 507 Model X cars, and shipped 208 of those, in the fourth quarter of 2015. Partly because of the low volume of Model X cars, Tesla’s shares dipped earlier this year.

According to the Wall Street Journal, Hoerbiger is seeking additional payment from Tesla for work it says it performed but wasn’t paid for. Tesla’s lawsuit asks the court to stop Hoerbiger from asking for more money, to declare that Tesla didn’t breach its contracts, and to seek damages and attorneys fees.

Will Tesla’s Model X be a hit?

Tesla and Musk are no stranger to legal disputes. Supplier Magna Powertrain sued Tesla in early 2008, after Tesla decided to use another company’s transmission for its first Roadster car (which was also delayed).

Tesla has been focused on increasing Model X production, and says it expects to produce 238 of them weekly now.