A howler in Harry Potter is an abusive letter that reads itself aloud, embarrassing the recipient. A howler in philosophy is a really bad mistake. If I'm about to commit a howler, don't send me any howlers.
Why don't I think that the Consequence Argument (in the kind of context in which Fischer is moved by it, ie, a context in which what is in question is alternative possibilities, and not sourcehood) is a problem for compatibilism? One reason is that it seems to be to be over-general: it doesn't tell us anything about determinism and alternative possibilities. Here's one way of getting at the point. Suppose you think the following argument, or some suitable refined version of it, is sound
|(P ⊃ Q)|
Then it seems to be that you should also find the following argument sound:
|(P ⊃ Q v R)|
|Therefore, Q v R|
But the second argument is just a simple version of the Consequence Argument for indeterministic worlds, and therefore allowing for alternative possibilities. In other words, the 'no choice' operator does not depend upon the assumption of determinism for any force it has.