How do I fix UBL-SR-39?

Your invoice contains multiple Project reference elements (ProjectReference/ID). EN16931 allows only one project reference per invoice. You can also use Invoice Navigator's compliance engine to correct this automatically in your pipeline.

Is UBL-SR-39 a critical error?

Yes, UBL-SR-39 is a critical error that will cause invoice rejection. It must be fixed before submission.

Can UBL-SR-39 be automatically fixed?

Many instances of UBL-SR-39 can be automatically corrected using Invoice Navigator's compliance engine. The fix is applied in your pipeline with full audit evidence.

errorEN16931

`UBL-SR-39`:Project reference shall occur maximum once.

Your invoice contains multiple Project reference elements (ProjectReference/ID). EN16931 allows only one project reference per invoice.

Severity

Fatal

Rule set

EN16931

Country

All EU

Fix type

CONFIRM

Confidence

90%

Engine Classification

Requires user confirmation before application

What is UBL-SR-39?

UBL-SR-39 is a fatal validation rule defined in the EN16931 specification. It validates the ProjectReference element in the UBL invoice XML.

When this rule fires, the invoice is non-compliant and will be rejected by Peppol access points and national validation services. The sending system receives a rejection response and the invoice does not reach the buyer.

Target path: /Invoice/cac:ProjectReference

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Why This Error Matters

Invoice will be rejected by EN16931 validators. These structural constraints ensure consistent processing by receiving systems.

UBL-SR-39 is a hard failure. Invoices that trigger this rule are rejected at the access point and never reach the recipient. In Peppol networks, this means your sending system receives an MLR (Message Level Response) with a rejection status. The invoice must be corrected and re-sent, adding delay to your payment cycle.

Validator Behavior

·Causes invoice rejection
·Fails EN 16931 business rule validation
·Error returned: UBL-SR-39
·Specification: EN16931

XML Example

Generic example based on the rule's target XPath. Your actual XML structure may differ.

Before

<!-- Required block is missing -->
<!-- <cac:ProjectReference> ... </cac:ProjectReference> -->

After

<!-- Add the required block -->
<cac:ProjectReference>
  <!-- child elements -->
</cac:ProjectReference>

Technical Reference

XPath/Invoice/cac:ProjectReference

SpecEN16931

Operationremove_duplicates

StrategyRemove duplicate project references, keeping the first occurrence

Common Causes

·ERP system exports multiple values where EN16931 only allows one
·Data consolidation merged records without deduplicating
·Template or mapping incorrectly configured to output duplicate elements
·Manual editing introduced extra elements
·Invoice conversion from another format did not merge duplicate entries

Seeing this in production? The API handles UBL-SR-39 automatically. See the fix response →

Handle UBL-SR-39 Automatically in Your Pipeline

The compliance engine auto-remediates this error with controlled safety policies and evidence pack generation.

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UBL-SR-39:Project reference shall occur maximum once.