Mongoose: Path is required at .findById()

I'm making a Blog with Node.js and a MongoDB (Mongoose) database in which users can share their blog.

I HAD a SubmittedBlog model like this:

const SubmittedBlogSchema = new mongoose.Schema({
    blogType: {
        type: String,
        required: true
    },
    text: {
        type: String,
        required: [true, 'Please enter a text'],
        minlength: [50, 'Text must be at least 50 characters long'],
        maxlength: [2000, 'Text must be at most 2000 characters long'],
        trim: true
    },
    ...
});

But then I had to add another field object called submitter which contains information about the blog submitter:

const SubmittedBlogSchema = new mongoose.Schema({
    blogType: {
        type: String,
        required: true
    },
    submitter: {
        id: {
            type: mongoose.Schema.Types.ObjectId,
            ref: "User",
            required: true
        },
        username: {
            type: String,
            required: true
        }
    },
    ...
});

Note that this object is required. However If I want to SubmittedBlog.findById() now, it isn't working and it errors out that "Path submitter.username is required". I thought that this would only happen at saving a submitted blog but why is this at finding a blog???