In Miller v Secretary of State for Exiting the European Union, the Supreme Court held that the UK Parliament was under no legal obligation to seek the consent of the Scottish Parliament before passing legislation to leave the European Union. Section 28(8) of the Scotland Act 1998, they held, does not elevate the Sewel Convention from the status of political convention to legal requirement. We argue that the Court’s reasoning on this point was unclear and underdeveloped, and betrayed a conceptual confusion about the nature of political conventions. We focus on four problematic assumptions that the Court made: (i) That a political convention does not cease being a political convention even if it is recognised in statute; (ii) That (i) obtains because of the nature of the content of a political convention, regardless of formal legal enactment procedures; (iii) That Parliament did not intend that a legislative provision should create a justiciable legal rule; (iv) That the words ‘it is recognised’ and ‘will not normally’ in the relevant provision support (iii). In making these assumptions, the court in effect created a new constitutional category: the ‘statutory convention’. We argue that this was misguided, as section 28(8) created a legal duty on the UK Parliament to seek the consent of the Scottish Parliament before passing legislation to trigger Article 50. This legislation has passed, but as the constitutional landscape becomes ever more complex, the confusion over the nature of political conventions created by the Court merits examination and conceptual clarification.