Genitive of animate nouns in Polish and Czech

When we study Slavic languages, we learn that genitives of animate nouns end in -a, but that doesn't always seem to be the case. Polish naród (nation) , szczep (tribe) and lud (folk/people) end in -u, which is common for inanimate nouns. The funny thing is that Czech národ has a genitive in -a, unlike Polish, but lid (Polish lud) follows a similar pattern: genitive lidu. Maybe it could be argued that animate collective nouns often have a -u genitive, but this has still to be researched.