How to extend trpc's default error codes?

I need to add more error codes to make error processing on the front-end easier. I tried adding a

trpc.d.ts

trpc.d.ts

in the types folder.

import { type TRPCError } from '@trpc/server';

declare module '@trpc/server' {
    interface TRPCError {
        code: 'TRIPS/SLUG_IN_USE';
    }
}

import { type TRPCError } from '@trpc/server';

declare module '@trpc/server' {
    interface TRPCError {
        code: 'TRIPS/SLUG_IN_USE';
    }
}

But it ain't working. How do you usually go about this?