Error when using custom validator

[oni-on1003]

I've got an error when using the custom validator isNumberLessThan21
Error: Cannot find a validator with a name isNumberLessThan21. This is a configuration error in your country specific resource package's form field definitions.

Usage:
export function testCustomValidator(...):SerializedFormField{.........,validator: [ { operation: 'isNumberLessThan21' } ] }