# DE-R-015

Source: https://xmlvalid.com/rules/DE-R-015

> If both supplier and customer are located in Germany, the element "Buyer reference" (BT-10) shall be provided.

- Level: error
- Rule sets: Peppol BIS Billing 3.0 rules 3.0.21
- Context: `(/ubl-invoice:Invoice | /ubl-creditnote:CreditNote)[$supplierCountryIsDE and $customerCountryIsDE]`
- Test: `cbc:BuyerReference[boolean(normalize-space(.))]`

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