Strange. It works for me. Try:
You made one of the most typical mistakes that our learners do - don't worry, it still happens a lot.
Basically, people are taking the rule too far and assume that there is such rule as "negation = Genitive". There is no such rule. The rule is actually "negated Accusative = Genitive". And as Accusative is the most common case, and also as some comments (including some of my older comments, probably) aren't specific enough, people assume that it's for all negations. But it's not. Only Accusative changes case when negated, everything else stays the same.