China’s foreign minister delivered a stark reminder to President-elect Trump Saturday. There is only one Chinese government to talk to.

The statement from the foreign minister’s office came after reports that Trump violated the “one China” policy and spoke on the phone with Taiwan’s president Friday. It was the only known time in nearly four decades that a a U.S. President has spoken on the phone with the president of Taiwan.

China views Taiwan’s president as an illegitimate leader, because they still view Taiwan as part of China. The U.S. has been delicately balancing that for more than 30 years.

“It must be pointed out that there is only one China in the world,” the Chinese foreign ministry said in a statement on Saturday, according to the Financial Times, which first reported the call between Trump and Taiwan’s president Tsai Ing-wen.

China’s foreign ministry added it “lodged solemn representations with the US.”

It’s still unclear if Trump’s call–in which Taiwan’s president congratulated him–was intended to cause the uproar and signal a new direction in U.S. foreign policy toward China or if Trump had been unaware of the precedent he was toppling.

“We urge the relevant side in the US to adhere to the ‘one China’ policy, abide by the pledges in the three joint China-US communiques, and handle issues related to Taiwan carefully and properly to avoid causing unnecessary interference to the overall China-U.S. relationship,” China’s foreign ministry stated.

Trump tried to rationalize his phone call Friday as it was coming under scrutiny.

“The President of Taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you,’ Trump tweeted.

He then later pointed out that the U.S. sells weapons to Taiwan.