Did you know that some masculine nouns in Genitive singular can end in у instead of а? These nouns are normally uncountable. This phenomenon is also known as Partitive case.
The Genitive case ending in у is more common in colloquial speech.
Я хочу немного винограду. I want some grapes.
Ты будешь чаю? Would you like some tea?
Он съел тарелку супу. He ate a plate of soup.