What or What a??

I read the grammatical rule that we use "what " where we have non countable things and "what a " where we can count things.
But at the same time we say
"what a nonsense !" How can we use "what a " here . we cant count nonsense. Similarly in many sentences where I think we should not use "what a" we still use it.
Is there something wrong with my interpretation of nonsense as a non countable thing?