In a recent contribution to legal ontology, Kenneth Ehrenberg identifies a puzzle concerning _the basic validity rule_ of legal systems: If formal institutions require a codified foundational constitutive rule, then legal systems cannot be formal institutions, since their foundational constitutive rule is necessarily an uncodified basic validity rule. To solve this puzzle, Ehrenberg suggests taking this rule as ‘a foundational and self-identifying institutional fact’. Here, I challenge his solution and the very existence of this puzzle. By arguing, contra Ehrenberg, that (...) the basic validity rule is not the foundational constitutive rule of a legal system, but only the foundational constitutive rule of legal validity; that its codification in fact contributes towards the institutionalisation of a legal system; and that the true foundational constitutive rule of a legal system is the formal group structure of the higher-constitutional organisations, I submit an alternative account of the institutional nature of legal systems. (shrink)