Transit gateway appliance mode support not supported?

I am trying to implement the TransitGateway Appliance VPC scenario from the documentation, using Terraform but run into trouble when I want to specify “appliance mode” for the vpc attachment to vpc-c:

resource "aws_vpc" "vpc_c" {
  cidr_block = ""

resource "aws_subnet" "subnets_in_vpc_c" {
  count = 2
  vpc_id     =
  cidr_block = cidrsubnet(aws_vpc.vpc_c.cidr_block, 8, count.index)

resource "aws_ec2_transit_gateway" "example" {}

resource "aws_ec2_transit_gateway_vpc_attachment" "vpc_c" {
  subnet_ids         = aws_subnet.subnets_in_vpc_c[*].id
  transit_gateway_id =
  vpc_id             =
  #appliance_mode_support = enable
  #dns_support = enable

output "appliance_mode" {
  value = aws_ec2_transit_gateway_vpc_attachment.vpc_c.appliance_mode_support

Uncommenting either appliance_mode_support or dns_support result in Terraform complaining about an “Invalid Reference” (even though those two arguments (and more) are perfectly ok according to the documentation)

Am I doing something completely wrong here?


Terraform v1.4.0-rc1
on darwin_arm64
+ provider v4.57.0
appliance_mode_support = "enable"


This format worked for aws_networkmanager_vpc_attachment
I suspect it may work for the TGW attachment as well

options { appliance_mode_support = true }