FIRInvalidArgumentException: Nested arrays are not supported

Question

Currently, I'm trying to use Swift's codable protocol to add and read custom objects to and from Firebase's database. I've been following the instructions in these docs. However, when I try to write to the database, I'm getting an error "FIRInvalidArgumentException: Nested arrays are not supported". But aren't they supported? I saw another post on here from a couple of years ago that addressed a similar issue but I don't understand what I did wrong here.

These are the Swift struct representations that implement Codable:

struct List: Codable {

    var title: String
    var date: String
    var description: String
    var data: [SectionHeaders: Array<Item>] = [
        .produce: [],
        .seafood: [],
        .meat: [],
        .deli: [],
        .bakery: [],
        .pantry: [],
        .dairy: [],
        .frozen: [],
        .other: []
    ]
    
    enum CodingKeys: String, CodingKey {
        case name
        case date
        case description
        case data = "items"
    }

struct Item: Codable {
    var name: String?
    var quantity: String?
}

enum SectionHeaders: Int, Codable {
    case produce = 0, seafood, meat, deli, bakery, pantry, dairy, frozen, other, total
    
    enum CodingKeys: String, CodingKey {
        case produce = "produce"
        case seafood = "seafood"
        case meat = "meat"
        case deli = "deli"
        case bakery = "bakery"
        case pantry = "pantry"
        case dairy = "dairy"
        case frozen = "frozen"
        case other = "other"
    }
}

And this is the data representation I'm aiming to get:

"title": String,
"date": String,
"description": String,
"items": [
     "header1": [{
          "name": String,
          "quantity": String
          },
          {
          "name": String,
          "quantity": String
          }
     ],
     "header2": [{
          "name": String,
          "quantity": String
          },
          {
          "name": String,
          "quantity": String
          }
     ]
]

Any help would be great! Thanks in advance!