Hey David. Some verbs don't follow the rule. one of those is （居）住 Other examples: 我住在新加坡 I live in Singapore 小明畫寫字在筆記本上 Xiao Ming writes characters on his notebook 這場火災發生在秘魯的雨林裡 This fire happened in the Peruvian rain forest 小王畫圖在紙上 Xiao Wang draws on the paper 小黃躺在床上 Xiao Huang lies on the bed

他在北京住。is right!