# next-intl Error: INVALID_MESSAGE: Missing defaultMessage

## TL;DR

Fixes the next-intl build error INVALID_MESSAGE caused by a useTranslations call with no defaultMessage and no matching key. Define the message in your locale files or pass a fallback. Use when the build fails naming INVALID_MESSAGE. Not for runtime missing-key warnings.

## The error

```text
Error: INVALID_MESSAGE: Missing `defaultMessage` for key `checkout.submit` in locale `en`.
```

## Fix it

### Step 1: Locate the offending translation call

```bash
grep -rn "checkout.submit" src app | head -10
```

Expected: You find the component calling useTranslations with that key.

### Step 2: Add the message to your locale files

```bash
node -e "console.log('add checkout.submit to messages/en.json and every other locale')"
```

Expected: The key exists in the messages source of truth.

### Step 3: Rebuild the project

```bash
npm run build
```

Expected: The build passes with no INVALID_MESSAGE error.

## When to use this

- The next-intl build fails with INVALID_MESSAGE
- A key is used in code but absent from every locale file

## When NOT to use this

- The app runs but shows the wrong text, that is a message content issue
- The error names a locale mismatch instead, check your locale routing

## Tool and version compatibility

- next-intl 3.x
- App Router with next-intl/plugin

## Variant phrasings

### error in development only

Dev mode tolerates missing messages; the build does not. Fix before shipping.

### many INVALID_MESSAGE errors at once

The messages file failed to merge. Check for JSON syntax errors before editing keys.

## Why it happens

next-intl validates at build time that every translated key resolves to a message. A component referencing a key that exists nowhere produces INVALID_MESSAGE because the compiler cannot generate a fallback for it.

## Edge cases

- Namespace keys like 'a.b.c' fail if an intermediate level is a string, not an object
- Locale files merged from multiple sources can silently drop keys on conflict
- Type-safe keys via createTranslator still error if the JSON is stale

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_3k6F2mfbjA-nNkav7T_OcQ