Well, if you don't want one, it's okay. But if it's a matter of "I don't want to put you out to getting one", that doesn't count as a reason not to get one. So...that is, if you say you really don't want one, I'll drop it, but be sure that if you say that, it's because the reason is "I don't want a cake", not "I don't think I deserve a cake".

[ She has to be sure which your refusal is meant to imply... ]