The Supreme Court’s judgment in Miller/Cherry  UKSC 41 holds that Parliamentary sovereignty needs to be judicially protected against the power of the Government to prorogue Parliament. But the Judgment itself undercuts the genuine sovereignty of Parliament by evading a statutory prohibition – art. 9 of the Bill of Rights 1689 – on judicial questioning of proceedings in Parliament. This paper shows that the Judgment was wholly unjustified by law. It wrongly transfers the conventions about prorogation into the domain of justiciable law. The Judgment is an inept foray into high politics and should be recognised as a historic mistake, not a victory for fundamental principle.