# BR-55 Preceding invoice reference must carry a reference identifier

Each Preceding Invoice reference (BG-3) shall contain a Preceding Invoice reference (BT-25).

## Problem Description

> **ERROR**: EN 16931 core business rule (CEN)

## Context and Significance

This is an EN 16931 core rule. A preceding invoice reference is mainly used in credit notes (381) and corrected invoices (384) to name the invoice being corrected; without its number (BT-25) the buyer cannot match the correction. The error appears when the reference group is created but only the date of the original invoice is exported, typically because the original invoice is not linked in the ERP.

## Solution

Add cac:BillingReference/cac:InvoiceDocumentReference/cbc:ID (CII: ram:InvoiceReferencedDocument/ram:IssuerAssignedID) with the number of the preceding invoice that is corrected or referenced.

- [Validate XML](/en/xrechnung-validator?source=resource&rule=BR-55)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&rule=BR-55)

## Valid Examples

- BT-25 = "INV-2026-0007" referencing the corrected invoice

## Invalid Examples

- cac:BillingReference without cac:InvoiceDocumentReference/cbc:ID, or a CII ram:InvoiceReferencedDocument with only ram:FormattedIssueDateTime and no ram:IssuerAssignedID

## Code Examples

### Valid XML
```
<ram:InvoiceReferencedDocument>
  <ram:IssuerAssignedID>INV-2026-0007</ram:IssuerAssignedID>
  <ram:FormattedIssueDateTime>
    <qdt:DateTimeString format="102">20260115</qdt:DateTimeString>
  </ram:FormattedIssueDateTime>
</ram:InvoiceReferencedDocument>
```

### Invalid XML
```
<ram:InvoiceReferencedDocument>
  <ram:FormattedIssueDateTime>
    <qdt:DateTimeString format="102">20260115</qdt:DateTimeString>
  </ram:FormattedIssueDateTime>
</ram:InvoiceReferencedDocument>
```

## Affected Fields

- [BT-25 Preceding Invoice reference](/resources/xrechnung/bt-25-preceding-invoice-reference)

## Related Rules

- [BR-52](/resources/kosit-validator-errors/br-52-supporting-document-reference-required)

## Validation Tools

- [Official KoSIT Validator](https://github.com/itplr-kosit/validator): Run the official KoSIT validator with the XRechnung configuration to check EN 16931 and XRechnung rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Find validation errors during conversion and review each correction.

## Related Resources

- [Official Specification](https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-tc434/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [All validation errors](/resources/kosit-validator-errors): Overview of the EN 16931, XRechnung and Peppol validation rules explained here.