message: better error for undefined fields
diff: @@ -24,7 +24,7 @@ module.exports = class ContentType {
  }
  }
  for (let [fieldName, field] of this.fields.entries()) {
  - if (!seen.get(fieldName)) {
  + if (field && !seen.get(fieldName)) {
  errors = errors.concat(await field.validationErrors(null, document));
  }
  }
  