Is it possible to use PropTypes to validate Dictionary-like objects?

I need to validate dictionary-like objects in my reducers, but since I'm already using Babel I don't want to resort to tools like Typescript.

Take this object as an example:

posts : {
    byId : {
        "post1" : {
            id : "post1",
            author : "user1",
            body : "......",
            comments : ["comment1", "comment2"]    
        },
        "post2" : {
            id : "post2",
            author : "user2",
            body : "......",
            comments : ["comment3", "comment4", "comment5"]    
        }
    }
    allIds : ["post1", "post2"]
}

How could I express my expectations for the byId object using PropTypes? Is it possible? If so, how?