It is no secret that a slowdown in China has affected Apple. But CEO Tim Cook said he made an error on the company's conference call by not discussing China's growing middle class.

"It was my error for not talking about it. I could not be more optimistic about China. I think the long-term thesis is intact," Cook said. (Tweet This)

In an interview with CNBC's Jim Cramer on Monday, Cook noted that the expansion of middle class occurring in China is unlike anything that has ever occurred in the history of the world, thus he remains optimistic in that region.

Cook explained that two years ago Apple produced enormous sales, and last year was even better, growing 80 percent from the previous year. This year's constant currency in mainland China was down 7 percent, so on a two-year basis; Apple grew about 70 percent in China.

"It's hard pressed to say those aren't good results," Cook said.

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Cook noted that he's thrilled about the launch of the iPhone SE and confirmed that services are strong. But he says the market in China is changing. As some of the heat in the smartphone market has died off, there is more movement between different suppliers of smartphones.

"I think in the areas where we play, we are doing quite well," Cook said.

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