Mongoose empty subdocument array

I made this mongoose schema with a nested array of subdocuments:

const deliverySchema = new db.Schema({
   price: Number
})

const suppliersSchema = new db.Schema({
    name: String,
    deliveries: [deliverySchema]
})

exports.Suppliers = db.model('Suppliers', suppliersSchema)

const suppliers = new Suppliers({name})

await suppliers.save()

But when I try to save a document i get this error:

TypeError: Cannot read properties of undefined (reading 'length')
    at cloneArray...

If I remove the subdocument from the schema the document gets saved without issues.

Why can’t I save the document with this schema?