In the fifth of his John Locke Lectures Robert Brandom takes up the challenge to define a formal semantics for modelling linguistic contents as established according to his normative analysis of linguistic practices. The project is to exploit the notion of incompatibility in order to directly define a modally robust relation of entailment. Unfortunately, it can be proved that in the original definition the modal system represented by Incompatibility Semantics collapses into propositional calculus. In the paper I show how IS can be technically amended so to overcome this failure: the required modifications are already known and consist in adapting and including the main notions of Kripke’s standard framework of possible worlds. I also show that the modifications do not jeopardize Brandom’s original project.