What mistke does Lieutenant Kotler made in conversation

When Bruno gets home, he finds out to his disappointment that Lieutenant Kotler is joining the family for dinner. Pavel waits on them, seeming to Bruno to be a bit weaker than usual. Bruno brings up that he hates learning about history and Father scolds him for it. Lieutenant Kotler offers that his father was a professor of literature at the university, but that he had left Germany for Switzerland in 1938. This information embarrasses Lieutenant Kotler and disturbs Father, who comments with suspicion, "Strange that he chose not to stay in the Fatherland" (146).