President-elect Donald Trump spoke with President Tsai Ing-wen of Taiwan on Friday, who offered her congratulations on his victory and discussed the close economic, political, and security ties between the two countries.

That might seem like an innocent courtesy, but apparently it is a topic of controversy between the United States and China, as any formal recognition of Taiwan severely angers the Chinese.

China views Taiwan as a province, as foreign policy experts were quick to point out, and the United States has severed diplomatic ties with Taiwan since 1979.

The Financial Times reports:

Although it is not clear if the Trump transition team intended the conversation to signal a broader change in US policy towards Taiwan, the call is likely to infuriate Beijing which regards the island as a renegade province.

The US has adopted the so-called “One China” policy since 1972 after the Nixon-Mao meetings and in 1978 President Jimmy Carter formally recognized Beijing as the sole government of China, with the US embassy closing in Taipei the year after.

According to a senior White House administration official speaking to the New York Times, they were not informed about the communications with Taiwan until after it happened.

Trump clarified on Twitter that the call was initiated by Taiwan and not by his campaign: