title: Error: Invalid template interpolation value: Cannot include the given value in a string template: string required
body: <p>I am writing a script for creating a kubernetes cluster in oracle cloud.
  When I run terraform plan, it returns no errors. But when I run terraform apply, it returns me the error:</p>
  
  <pre><code>Error: Invalid template interpolation value: Cannot include the given value in a string template: string required.
  </code></pre>
  
  <p>I enabled trace and log file generation to try to identify where the problem is, in trace it returns:</p>
  
  <pre><code>2019/11/21 20:31:15 [TRACE] EvalMaybeTainted: null_resource.k8sworker-ad1 [0] encountered an error during creation, so it is now marked as tainted
  2019/11/21 20:31:15 [ERROR] : eval: * terraform.EvalApplyPost, err: 1 error occurred:
  
      Invalid template interpolation value: Cannot include the given value in a template: string required string.
  
  2019/11/21 20:31:15 [TRACE] EvalMaybeTainted: null_resource.k8sworker-ad1 [1] encountered an error during creation, so it is now marked as tainted
  2019/11/21 20:31:15 [ERROR] : eval: * terraform.EvalSequence, err: Invalid template interpolation value: Cannot include the given value in a string template: string required.
  </code></pre>
  
  <p>I believe the problem is in this part of the code:</p>
  
  <pre><code>resource "null_resource" "k8sworker-ad1" {
    count      = var.k8sWorkerAd1Count
    depends_on = [module.instances-k8sworker-ad1]
  
    triggers = {
      worker_id       = module.instances-k8sworker-ad1.ids[0][count.index]
      build_source_id = null_resource.build_source.id
    }
  
    provisioner "local-exec" {
      command = "echo 'alias ${var.label_prefix}workerad1-${count.index}=\"ssh -i ${path.root}/generated/instances_id_rsa opc@${element(module.instances-k8sworker-ad1.public_ips, count.index)}\"' &gt;&gt; source.sh"
    }
  }
  </code></pre>
  
  <p>The Terraform Version is:</p>
  
  <pre><code>Terraform v0.12.16
  + provider.null v2.1.2
  + provider.oci v3.52.0
  + provider.random v2.2.1
  + provider.template v2.1.2
  + provider.tls v2.1.1
  </code></pre>
  
  <p>The trace output is:</p>
  
  <p><a href="https://gist.github.com/RuyCury/bc0dbccc65bbbadf2eb90569fd438286" rel="nofollow noreferrer">https://gist.github.com/RuyCury/bc0dbccc65bbbadf2eb90569fd438286</a></p>
  
  <p>Please help me solve this problem.</p>