"There isn't" or the absence of something always takes the genitive never the accusative. There are exceptions for other meanings too.
Мы не достигли этого [genitive]. "We didn't achieve that."
Он мне не дал деньги [accusative]. "He didn't give me money." Он мне не дал денег [genitive]. "He didn't give me any money," or "the money."
Моя бабушка не хотела перца [genitive]. "My grandmother didn't want any pepper."