Well you always know that one of your counterfactuals is true.
There is no need to make that assumption. The whole collection of possible decisions could be located on an impossible counterfactual. Incidentally, this is one way of making sense of Transparent Newcomb.
Would you ever actually be in a situation where you chose an action tied to an impossible counterfactual? Wouldn't that represent a failure of Omega's prediction?
And since you always choose an action...
Y'all know the rules: