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. You can also use Invoice Navigator's automatic Fixer tool to correct this instantly.
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.
Many instances of KSEF-001 can be automatically corrected using Invoice Navigator's Fixer tool. Upload your invoice and the tool will identify and fix common issues, including nip validation failed.
Last updated: 19 January 2026
Handle KSEF-001 Automatically in Your Pipeline
The compliance engine auto-remediates this error with controlled safety policies and evidence pack generation.