The Jet Black iPhone 7 and iPhone 7 Plus are the most popular new iPhones this year — but that doesn’t mean you’ll get yours anytime soon. Apparently, the Jet Black color isn’t the easiest to manufacture, and Apple has run into various production problems that still haven’t completely been ironed out.

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Well-connected Apple insider Ming-Chi Kuo said in a note to investors obtained by Business Insider that mass-producing large quantities of Jet Black iPhones is still problematic for iPhone.

The Jet Black iPhone model suffers “from a low casing production yield rate of 60-70%” according to Kuo. That means suppliers are producing a lot of metal shells that don’t meet Apple’s quality standards. It’s not clear what the problems are, but it’s certainly not surprising to hear that Jet Black iPhones are harder to make than any other color.

The analyst also pointed out a second factor affecting Jet Black iPhone availability: Jet Black is the most popular choice for consumers. A preorder survey conducted by the firm showed that Jet Black iPhones accounted for 30-35% of preorders worldwide. In China alone, almost 50% of iPhones 7 preorders were Jet Black.

“The survey offers an insight into iPhone buyers’ tendency to choose models that are aesthetically different from the 6S. Popularity of the Jet Black version indicates that consumers prefer an alternative to the traditional metal finish,” Kuo wrote.

The survey also showed that 50% of preorders went to the iPhone 7 Plus.

Overall, Kuo believes that Apple will sell fewer iPhone 7 models in 2016 than iPhone 6s units in 2015. The analyst said Apple could sell 70 million to 75 million iPhone 7 units this year, up from his previous estimate of 65 million units.