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# SKAdTestError.Code.invalidSourceIdentifier

The postback’s identifier isn’t two, three, or four digits.

```
case invalidSourceIdentifier
```

## Discussion

A valid postback includes two to four digits of the impression’s <doc://com.apple.documentation/documentation/StoreKit/SKAdImpression/sourceIdentifier>. For more information about the varying length of source identifiers, see <doc://com.apple.documentation/documentation/StoreKit/receiving-postbacks-in-multiple-conversion-windows>.

---

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