title: Error with Subnet ID while creating EC2 instaces with module
body: <p>I am trying to create 3 ec2 instances in private subnet. Using ec2-module.tf from below git repo</p>
  
  <p><a href="https://github.com/quickbooks2018/Terraform-Classic-Modules" rel="nofollow noreferrer">https://github.com/quickbooks2018/Terraform-Classic-Modules</a></p>
  
  <p>Below is the line I enabled in tf file</p>
  
  <p>ec2-subnets-ids = ["${module.vpc.private-subnet-ids}"]</p>
  
  <p>Error looks below, </p>
  
  <p>Error: Invalid value for module argument</p>
  
  <p>on ec2-module.tf line 17, in module "ec2-app-v1":
    17:   ec2-subnets-ids = ["${module.vpc.private-subnet-ids}"]</p>
  
  <p>The given value is not suitable for child module variable "ec2-subnets-ids"
  defined at modules/ec2/ec2-variables.tf:46,1-27: element 0: string required.</p>
  
  <p>Please help to understand the error and how to fix it.</p>