CoreData project error NULL _cd_rawData but the object is not being turned into a fault

Question

I receive the error CoreData: fault: NULL _cd_rawData but the object is not being turned into a fault. I have looked at many questions related to this issue and they all reference thread issue, but I am doing everything on the main thread using NSPersistentContainer.viewContext.

The problem occurs when I reference an Entity attribute that is a transformable type that is a Swift array of type [Double] the first time. Subsequent accesses do not show the error. The error also does not create an overall program fault and execution continues.

Since many other questions suggest this is a thread concurrency issue, I run the program with '-com.apple.CoreData.ConcurrencyDebug 1' but when the error appears in the console no expected errors are raised from the ConcurrencyDebug flag.

The code for my fetch request is below.

I set a relation programmatically between SMLocation and FlatPricing. The array attributes of FlatPricing have the problem, but only for the first time I access a value of the attribute.

public class FlatPricing: NSManagedObject {
    ....

    @NSManaged public var monthlyRate: [Double]? 
    @NSManaged public var monthlyUseCap: [Double]? 
    @NSManaged public var householdsAtCap: [Int]?

   ....
 }


func fetchThenSetManagedObjects(forLocation loc: String) -> SMLocation? {

        let fetchReq: NSFetchRequest<SMLocation> = SMLocation.fetchRequest()
        fetchReq.resultType = .managedObjectResultType
        fetchReq.predicate = NSPredicate(format: "smlName == %@",loc)

        var fetchRes: [SMLocation] = [SMLocation]()
        do {
            fetchRes = try persistentContainer.viewContext.fetch(fetchReq)
        } catch {
            print("Core Data Fetch failed")
            print(error)
            return nil
        }

        // now set all the managed objects
        return fetchRes[0]

    }

I set the FlatPricing object like this in various parts in my code:

....

guard let smLoc = fetchThenSetManagedObjects(forLocation: currentLocation) else {
       print("Fetch failed for locationSelect")
        return
    }
let initFlatPricing = (smLoc.value(forKey: "flatPricing") as! FlatPricing)

When I access an attribute like:

guard var h = initFlatPricing.householdsAtCap else {
    return
}

h[0] = 12

The assignment triggers the error. But a subsequent assignment to that array attribute or any of the other array attributes works fine.