President Trump tweeted that North Korean leader Kim Jong Un is in “good health” a day after journalist Bob Woodward unveiled details about their relationship.

“Kim Jong Un is in good health. Never underestimate him!” Trump said in a Thursday morning tweet.

It is unclear what exactly prompted the president to speak out about the well-being of North Korea’s dictator. “I feel pleased to have formed good ties with such a powerful and preeminent statesman as Your Excellency,” Kim said in another letter.