The fallacy of exclusive premises is a formal fallacy committed in a categorical syllogism that is invalid because both of its premises are negative. A logical fallacy is an error in logical argument which is independent of the truth of the premises. ... Wikipedia does not yet have an article with this exact name. ... In logic, an argument is said to be valid if the truth of the conclusion follows from the truth of the premises. ...

Example:

Since no mammals are fish and some fish are not whales, it follows that some whales are not mammals.

Note: That the second premise is wrong because "No fish are whales" not "some fish are not whales."