Okay, i tried to say "You can't think of a single russian joke?" and this is what I said:

не знаешь одну русскую шутку?

But I was corrected, and apparently the right way to say this is:

не знаешь ни одной русской шутки?

My question is, it seems like одной и русской are in the prepositional, but шутки is in just plural (or accusative plural). Why is this? I thought adjectives always had to agree in case, but it doesn't seem like that here. What am I missing? Thanks,

tdk