[TypeScript] Incorrectly allowing any key/value pair on a typed object

[beefchimi]

While using reduce(), TypeScript is allowing me to add any key/value pair to the object:

export function filterNetworkProperties(
  network: SocialNetwork,
  subset: SocialNetworkProperties,
) {
  const uniqueProperties: SocialNetworkProperties = [...new Set(subset)];
  const keys = Object.keys(network) as SocialNetworkProperties;

  return uniqueProperties.length
    ? keys.reduce<SocialNetworkSubset>(
        (filtered, property) =>
          uniqueProperties.includes(property)
            ? {
                ...filtered,
                [property]: network[property],
                // BUG: This should not be allowed!
                foo: 'bar',
              }
            : filtered,
        {} as SocialNetworkSubset,
      )
    : network;
}