Apple Inc. CEO Tim Cook attends China Development Forum (CDF) 2018 at the Diaoyutai State Guesthouse on March 24, 2018 in Beijing, China. China Development Forum (CDF) 2018 is hosted by the Development Research Center of the State Council of China on March 24-26 in Beijing.

Goldman Sachs estimates Apple will produce 6 percent fewer iPhones next year than previously expected, after key supplier Lumentum reduced its shipment outlook, according to a report Tuesday.

"We are concerned that end demand for new iPhone models is deteriorating," Goldman said in the note. "We note this could easily right itself given the bulk of demand comes in late December but we feel more prudent sell through forecasts are warranted due to the timing and magnitude of this warning."

WATCH: Apple's stock is plunging — Here's what six experts say investors should know