Apollo readFragment and dataIdFromObject

Question

I've configured ApolloClient like this:

import { ApolloClient, InMemoryCache } from '@apollo/client';

export const apolloClient = new ApolloClient({
    uri: 'http://localhost:4000',
    cache: new InMemoryCache({
        dataIdFromObject: obj => obj.id as string,
        addTypename: false
    })
});

Now I want to use apolloClient.readFragment to read something from the cache, however it keeps returning an empty object {} even though it looks like it's in the cache.

            console.log('readFragment',optimisticUpdate)
            const {id,fragment,data} = optimisticUpdate
            rollback = apolloClient.readFragment({id,fragment},true)
            console.log("ROLLBACK",rollback)
            apolloClient.writeFragment(optimisticUpdate)

If I remove the dataIdFromObject and addTypename options from InMemoryCache and change the id to include the typename then it seems to work, but I don't see why I should have to do that because all my IDs are guaranteed to be unique.