A line period has neither a start nor an end date
As soon as an invoice line period (BG-26) is present, it must carry its start date (BT-134), its end date (BT-135), or both.
Only the issuer holds this value: Factlint reports it and never fabricates it. Test your invoice to find out whether it is affected.
The message your validator prints
Copied verbatim from the sealed artefact: this is the exact sentence the check prints when this rule fails.
[BR-CO-20]-If Invoice line period (BG-26) is used, the Invoice line period start date (BT-134) or the Invoice line period end date (BT-135) shall be filled, or both.
Quoted from schematron-cen 1.3.16, licensed under EUPL-1.2.
Common searches around this code
These phrases mirror what an integrator searches for: code, syntax, rule, business field and correction.
Test your file with this error in mind
Upload the invoice involved: the report will say whether this code appears, where it appears in the document, and whether Factlint can only report it or also fix it.
To automate this check in an ERP flow, call the validation API and link every returned code to its public page.
Why this error happens
Same mechanics as at document level, but far more frequent: the line period is often copied from the invoicing period, and the group ends up written even when the source is empty. A single affected line is enough to fail the document.
How to fix it
Fill one bound on the reported line, or write the group only when it carries data. Prefer fixing the serialiser: the case will recur on every invoice built from the same template.
XML correction example
<ram:BillingSpecifiedPeriod/>
<ram:BillingSpecifiedPeriod>
<ram:EndDateTime>
<udt:DateTimeString format="102">20260831</udt:DateTimeString>
</ram:EndDateTime>
</ram:BillingSpecifiedPeriod>