Since the verb stem of the negative copula 아니다 ends in in a vowel (아니다 => 아니), some people add 예요 at the end of the verb stem : 아니예요. The correct form should be 아니에요. (verb stem 아니 + 에요) like for the copula 이다 (verb stem 이 + 에요 => 이에요). As far as I know no one writes 이예요! The confusion comes from the fact that the polite copula is written -예요 after a a vowel although in colloquial Korean is pronounced [-에요) e.g 할 거예요 [할꺼에요]

In terms of grammar, "아니에요" is correct, but people are confused to tell them often, even Koreans. It's because "니" in "아니" doesn't have a final consonant(=받침). \^o^/