Christine Lagarde doesn’t mince words often, but when she does, bond traders tend to listen. The European Central Bank president has come out swinging against a proposal from French far-left leader Jean-Luc Mélenchon to simply erase the portion of France’s debt sitting on central bank balance sheets, calling the idea financially dangerous and fundamentally incompatible with how the Eurozone works.

The target in question: roughly €600 to €636 billion in French sovereign debt held by the Eurosystem. That’s approximately 18% of France’s total debt stock, which currently sits at about €3.536 trillion, or 117.5% of GDP. Mélenchon’s pitch is elegantly simple on the surface. If the central bank owns the debt, just cancel it and free up fiscal space for spending. The problem, according to Lagarde, is that elegantly simple and legally possible are two very different things.

Why the ECB says this breaks the rules

At the heart of Lagarde’s objection is Article 123 of the Treaty on the Functioning of the European Union. That provision explicitly prohibits monetary financing of governments, which is a fancy way of saying central banks can’t just print money to cover national budgets. Canceling debt held by the Eurosystem would effectively do exactly that, turning what were market transactions under quantitative easing into direct fiscal subsidies.