The illegality defence is an important element of private law, but its operation has been unpredictable. In Patel v Mirza, the Supreme Court opted for a flexible approach, which does not increase predictability. This approach was recently confirmed in Henderson v Dorset Healthcare University NHS Foundation Trust and Stoffel & Co v Grondona. I propose a principle to guide the application of the illegality defence in tort: namely, the claim fails for illegality if the claimant's harm is the ordinary result of the claimant's wrongdoing. It is argued that the guiding principle: (i) substantially explains the case law; (ii) is normatively defensible; and (iii) makes the defence much more predictable.