VOW-ERR-11 VIES Error: Cause and Fix

VOW-ERR-11 is a VIES REST API error with the message "A mandatory field is missing". First check that your request sets countryCode and vatNumber. If it does, handle it as an availability error: the number was not checked, so retry with backoff and never mark it invalid.

A mandatory field is missing
VIES REST API error message, as recorded by Avatcado on 2026-09-04

What it means

The VIES REST API reports errors in an errorWrappers list, and each entry carries an error code and a message. VOW-ERR-11 arrives with the message "A mandatory field is missing", the same text VIES on-the-web uses for a missing field. The fault list in the SOAP WSDL does not include it.

What causes it

On 2026-09-04 Avatcado saw Austria, the Czech Republic and France answer complete requests with HTTP 200, actionSucceed false and VOW-ERR-11, while the VIES check-status service reported the Czech Republic as unavailable. In that outage a member state failure came back as a validation error, and Avatcado treats VOW-ERR-11 as an availability error for that reason.

How to handle it

Check your request first and make sure countryCode and vatNumber are both set.
If the request is complete, treat the result as unknown, never as invalid.
Look the member state up in GET /check-status on the VIES REST API.
Retry with backoff, stop after a bounded number of attempts, then queue the check.

What Avatcado does when VIES returns VOW-ERR-11

Avatcado retries VOW-ERR-11 inside the same request, with a short pause and random jitter between attempts.

If VIES still has no answer, Avatcado asks the country's national tax register on the Pro and Business plans, for the 10 EU countries that have one (listed on the coverage page), and on every plan it falls back to the most recent stored result for that number. When neither can answer, the API returns a 503 with the code upstream_member_state_unavailable, and the call does not count against your quota. A check that ends in an availability error is never stored as an answer. A request that carries requester_vat_number skips both fallbacks by default, because a consultation number has to come from a completed VIES check.

How often Avatcado sees VOW-ERR-11

Frequently asked questions

Why does VIES say a mandatory field is missing when my request is complete?

In the case Avatcado recorded on 2026-09-04, complete requests for Austria, the Czech Republic and France came back with VOW-ERR-11 while VIES's own status service listed the Czech Republic as unavailable. Avatcado read that as a member state failure reported through a validation error. When your request sets both countryCode and vatNumber, retry it later before concluding anything about the number.

Does Avatcado retry VOW-ERR-11?

Yes. VOW-ERR-11 is in Avatcado's set of retryable VIES errors, so a check that hits it is repeated inside the same request. If VIES keeps returning it, the request moves on to the national register on the Pro and Business plans, for the countries that have one, then to the most recent stored result, and ends in a 503 that does not count against your quota when neither can answer.

VOW-ERR-11 VIES error: what it means and what to do