# The error `value.unknown-part`

**user**: the script can change it: the error says what happened in the experiment's names, and what to write instead.

**On the refusal list: no test names this code**, since no file under `tests/` names it. A test may still raise it without naming it, and none asserts it: what its message and action say is read from the source below. The hook that records which codes a test raises has not landed, so until it does the list says names and never raises. The list (`docs/ratchets/refusal_codes.txt`) only shrinks.

Where MIME gives it, with what its message says happened and what its action says to do (`{…}` is filled in from the experiment):

- in `mime/materials/_values.py`:
  - what happened: `the declared value {…} names no part of a body or an apparatus`
  - what to do: `name one of its parts, exp.declare("<thing>.parts.<part>.<value>", ...). {…}` or `{…} declares no parts: give it parts={...}`

Reading an error and reaching its full traceback: [reading an error](../reading_errors.md). Every code: [the index](index.md).