I’m probably holding it wrong

If I wanted to do a table view data source in ObjC, it would look like this:

- tableView:aTableView objectValueForTableColumn:aColumn row:(NSInteger)row {
  return [representedObject.collection[row] valueForKey:[aColumn identifier]];
}

When I do it in Swift, it ends up looking like this:

func tableView(_ tableView: NSTableView, objectValueFor tableColumn: NSTableColumn?, row: Int) -> Any? {
    guard let identifier = tableColumn?.identifier else {
        assertionFailure("No table column")
        return nil
    }
    guard let obj = (self.representedObject as? ModelType)?.collection(at:row) else {
        assertionFailure("Can't find model object at \(row)")
        return nil
    }
    switch identifier {
    case NSUserInterfaceItemIdentifier(rawValue:"column1"):
        return obj.field1
    case NSUserInterfaceItemIdentifier(rawValue:"column2"):
        return objc.field2
    //...
    default:
        assertionFailure("Unknown table column \(tableColumn?.identifier ?? NSUserInterfaceItemIdentifier(rawValue: "unknown"))")
        return nil
    }
}

I can’t help feeling I’m doing it wrong.