title: Unable to add 2 subnets for an ElasticSearch with Terraform
body: <p>I'm trying to build an ElasticSearch cluster using Terraform but i'm not able to assign more that 1 subnet! That's really weird cause in the documentation there is this : </p>
  
  <p><a href="https://www.terraform.io/docs/providers/aws/r/elasticsearch_domain.html#subnet_ids" rel="noreferrer">https://www.terraform.io/docs/providers/aws/r/elasticsearch_domain.html#subnet_ids</a></p>
  
  <blockquote>
    <p>subnet_ids - (Required) List of VPC Subnet IDs for the Elasticsearch
    domain endpoints to be created in.</p>
  </blockquote>
  
  <p>But when i try to do that i'm getting this error : </p>
  
  <blockquote>
    <p><strong>Error: ValidationException: You must specify exactly one subnet</strong></p>
  </blockquote>
  
  <p>This is my code : </p>
  
  <pre><code>resource "aws_elasticsearch_domain" "es" {
    domain_name           = "${var.es_domain}-${var.environment}"
    elasticsearch_version = "${var.es_version}"
  
    cluster_config {
      instance_type  = "${var.es_instance_type}"
      instance_count = "${var.es_instance_count}"
    }
    vpc_options {
  
      subnet_ids = ["${data.aws_subnet.private_1.id}", "${data.aws_subnet.private_2.id}"]
  
      security_group_ids = ["${aws_security_group.es.id}"]
    }
  
    snapshot_options { automated_snapshot_start_hour = "${var.es_automated_spanshot_start_hour}" }
  
    ebs_options {
      ebs_enabled = true
      volume_type = "standard"
      volume_size = "20"
    }
  
  
    access_policies = &lt;&lt;CONFIG
      {
      "Version": "2012-10-17",
      "Statement": [
        {
          "Action": "es:*",
          "Principal": "*",
          "Effect": "Allow",
          "Resource": "arn:aws:es:${var.aws_region}:${data.aws_caller_identity.current.account_id}:domain/${var.es_domain}/*"
        }
      ]
  }
  CONFIG
  
  
  }
  </code></pre>
  
  <p>I'm using terraform v0.12.2</p>
  
  <p>Thanks for your help.</p>