Funny enough I have my own reasons for having thought he was Croatian. A few years ago I visited my ancestral village in Italy's Abruzzo region and there happened to be a street named after Tesla... I asked about it and learned that Tesla was from nearby, just across the Adriatic in Croatia. Hence in my head it was cemented.

Really I suppose I got the wrong idea in my head because Italians are much more familiar with their naval neighbor Croatia than further-away Serbia :)