KSEF-001:NIP validation failed
Polish NIP (tax identification number) must be 10 digits with valid checksum. Both seller and buyer NIP are required for B2B invoices. Leading zeros are significant.
Engine Classification
Business data required · Explicit input workflow · No assumptions made
Required input: Polish NIP (Tax ID)
What is KSEF-001?
KSEF-001 is a fatal validation rule defined in the KSeF specification (POL national rules). It checks whether nip validation failed.
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.
Why This Error Matters
KSEF-001 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
- ·Error returned: KSEF-001
- ·Specification: KSeF
How to Fix It
Technical Reference
Code Example
<NIP>1234567890</NIP>Common Causes
- ·NIP entered with only 9 digits
- ·Invalid checksum
- ·NIP cancelled or inactive
Seeing this in production? The API handles KSEF-001 automatically. See the fix response →
Frequently Asked Questions
Polish NIP (tax identification number) must be 10 digits with valid checksum. Both seller and buyer NIP are required for B2B invoices. Leading zeros are significant.
Verify the NIP is 10 digits with valid checksum. Invoice Navigator detects this error and prompts for the required business data before applying a safe fix.
Yes, KSEF-001 is a critical error that will cause invoice rejection. It must be fixed before submission to ensure your invoice is accepted by the recipient's system.
KSEF-001 requires specific business data to resolve. Invoice Navigator detects the error and guides you through providing the needed information, then applies the fix with full audit documentation.
Related Content
Last updated: 3 March 2026
Resolve KSEF-001 with Guided Input
The compliance engine detects this error and prompts for the required business data before applying a safe, auditable fix.