It's certainly genitive. Some prepositions can be a little bit tricky. One of those prepositions is "do". After "do" we always use nouns in genitive.

Jordan is right, it's genetivus.

It's Genitive with preposition 'do': - Dokąd idziesz? - Idę do sklepu, do domu, do biura, do pracy, do mamy, do ciebie, do was, do kogoś, etc.

Pretty sure it's genative. i know that after 'do' is always -u ending with masculine inanimate nouns. That's all the info i can give you, really. I'm trying to figure it out as well. http://www.polish-dictionary.com/polish-genitive-case