Inappropriate value for attribute "your_variable": set of string required.

dependency "vpc" {
  config_path = "../vpc"
  mock_outputs = {
    private_route_table_ids = ["temporary-dummy-id"]
  }
}

inputs = {
  private_route_table_ids = dependency.vpc.outputs.private_route_table_ids
}

variable "private_route_table_ids" {}

$ terragrunt run-all plan

Error: Incorrect attribute value type
  on xxxx.tf line 10, in resource "xxxx" "xx":
  10:   route_table_ids = var.private_route_table_ids
    |----------------
    | var.private_route_table_ids is “[\“temporary-dummy-id\“]”
Inappropriate value for attribute “route_table_ids”: set of string required.

variable "private_route_table_ids" {
  type = set(string)
}