How to access the request body in the onError callback?

TRPCError: "input" needs to be an object when doing a batch call

fetchRequestHandler({
    endpoint: '/api/trpc',
    req,
    router: appRouter,
    createContext: () => ({}),
    onError: async ({ error, req, input }) => {
      console.log('TRPC route error', error);

      let body = null;

      try {
        if (req.method === 'POST') {
          const isJsonContentType =
            !!req.headers.get('Content-Type')?.includes('application/json') ??
            false;
  
          body = isJsonContentType ? await req.json() : await req.text();
        }
      } catch () {
        // here it would throw that the body is unusable
      }
       
      console.log(
        req.method,
        req.headers.get('Content-Type'),
        { body, input },
       // here: both, body and input are empty 
      );

      // ...
    }
}