There is a small issue with the exported types. Doing a import { ERROR } from 'feathers-objection' throws a typescript error because the index.d.ts file doesn't include it. The symbol is still imported and everything works fine with a // @ts-ignore directive.

Edit:

I think we just need to change the declare const objection bit to something more like

declare const objection: {
  <T>(config?: Partial<ObjectionServiceOptions>): Service<T>
  ERROR: Symbol
}

https://www.typescriptlang.org/docs/handbook/interfaces.html#hybrid-types