# D021 postcondition_violated

URL: /docs/errors/D021
Markdown: /docs/errors/D021.md

> An ensures clause failed on normal return.

A runtime defect: the running program stops with it; no compile phase reports it, and `try` cannot catch it. `sakura explain D021` prints this explanation offline.

An `ensures` clause failed on normal return. The callee is blamed: fix the function body, or
weaken the clause if it promises more than the function can deliver. `sakura test --contracts`
and `sakura verify` look for such inputs before the program runs.

Runtime defects come from running programs, so `compiler/tests/ui` has no fixture for them; the explanation above names the repair.