In its judgment of May 5, 2020, the Second Senate of the German Constitutional Court qualified for the first time a judgment of the Court of Justice of the European Union (CJEU) – C-493/17 Weiss – as “arbitrary from an objective perspective” and declared the underlying European Central Bank (ECB) decisions regarding the Public Sector Purchase Programme (PSPP) to be ultra vires. It requested the German Government and the German Parliament to take steps against the PSPP in its current form and to ensure that the ECB conducts a proportionality assessment of its PSPP. The judgment also prohibits the German Central Bank from participating, after a grace period of three months, in the implementation and execution of the ECB decisions at issue, unless the ECB assesses and substantiates that the measures provided for in its decisions satisfy the principle of proportionality. The present article, which was written by academics, lawyers, and civil servants from five countries, casts a critical eye on the judgment of the German Constitutional Court. It identifies significant shortcomings from both a German constitutional and a European Union perspective.