Unable to use with existing VPC

[shadycuz]

I'm trying to use this module to add transit subnets and attachment to my existing VPC.

module "vpc" {
  source   = "aws-ia/vpc/aws"
  version = ">= 4.4.2"

  name                                 = "sandbox"
  # cidr_block                           = "10.80.0.0/16"

  vpc_id = "vpc-xxxxxxxx"
  vpc_assign_generated_ipv6_cidr_block = false
  vpc_egress_only_internet_gateway     = false
  az_count                             = 3

  transit_gateway_id = "tgw-xxxxxx"

  subnets = {
    public = {
      name_prefix               = "sandbox-public" # omit to prefix with "public"
      nat_gateway_configuration = "none" # options: "single_az", "none"
      assign_ipv6_cidr         = false
      cidrs      = ["10.80.144.0/23", "10.80.176.0/23", "10.80.208.0/23"]
    }
    # IPv4 only subnet
    private = {
      name_prefix  = "sandbox-private"
      connect_to_public_natgw = false
      assign_ipv6_cidr         = false
      cidrs = ["10.80.128.0/23","10.80.160.0/20","10.80.192.0/20"]
    }

    transit_gateway = {
        name_prefix = "sandbox-transit-gateway"
        assign_ipv6_cidr                                = false
        transit_gateway_default_route_table_association = false
        transit_gateway_default_route_table_propagation = false
        transit_gateway_appliance_mode_support          = "enable"
        transit_gateway_dns_support                     = "disable"
        cidrs = ["10.80.146.0/28", "10.80.178.0/28", "10.80.210.0/28"]
    }
  }
}

If I supply the VPC ID as vpc_id and run plan I get the following:

Plan: 16 to add, 0 to change, 0 to destroy.
╷
│ Error: Invalid index
│ 
│   on .terraform/modules/sandbox_us_east.vpc.calculate_subnets_ipv6.subnet_calculator/main.tf line 2, in locals:
│    2:   cidr_netmask = tonumber(split("/", var.base_cidr_block)[1])
│     ├────────────────
│     │ var.base_cidr_block is ""
│ 
│ The given key does not identify an element in this collection value: the given index is greater than or equal to the length of the collection.

If I remove vpc_id and use cidr_block, I get the same error when trying to import the existing VPC.

tofu import module.sandbox_us_east.module.vpc.aws_vpc.main[0] vpc-xxxxxx
...
module.sandbox_us_east.module.vpc.aws_vpc.main[0]: Import prepared!
  Prepared aws_vpc for import
module.sandbox_us_east.module.vpc.aws_vpc.main[0]: Refreshing state... [id=vpc-xxxxxxx]
...

│ Error: Invalid index
│ 
│   on .terraform/modules/sandbox_us_east.vpc.calculate_subnets_ipv6.subnet_calculator/main.tf line 2, in locals:
│    2:   cidr_netmask = tonumber(split("/", var.base_cidr_block)[1])
│     ├────────────────
│     │ var.base_cidr_block is ""
│ 
│ The given key does not identify an element in this collection value: the given index is greater than or equal to the length of the collection.

I am using tofu instead of terraform, but I didn't think that was the problem.