TypeScript `|` Combinator

I have the following code:
ceramic.did = new DID({
  provider: threeID.getDidProvider(),
  resolver: {
    ...get3IDResolver(ceramic), // returns a ResolverRegistry
    ...getKeyResolver(),        // returns a ResolverRegistry
  } as ResolverRegistry,
});

And, TypeScript is giving me the following error:
Type 'ResolverRegistry' is not assignable to type 'Resolver | ResolverRegistry | undefined'.
  Type 'ResolverRegistry' is missing the following properties from type 'Resolver': registry, cache, resolvets(2322)
(property) resolver?: Resolver | ResolverRegistry | undefined

I thought that let pet: Cat | Dog means I can assign either a Cat or Dog to it. Is that not right?
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