Full Idea
It is contentious whether conditionals have negations, and whether 'it is not the case that if A,B' has any clear meaning.
Gist of Idea
It is doubtful whether the negation of a conditional has any clear meaning
Source
John P. Burgess (Philosophical Logic [2009], 4.9)
Book Reference
Burgess,John P.: 'Philosophical Logic' [Princeton 2009], p.96
A Reaction
This seems to be connected to Lewis's proof that a probability conditional cannot be reduced to a single proposition. If a conditional only applies to A-worlds, it is not surprising that its meaning gets lost when it leaves that world.