terraform random_string keepers attribute not found

I'm trying to use the random_string terraform resource with keepers attributes like so.

resource "random_string" "rand" {
  length = 16
  special = false
  upper = false

  keepers = {
    md5 = "${md5(file("./terraform/batch.tf"))}"
    service_role = "arn:aws:iam::${var.target_aws_account_id}:role/BatchService"
    instance_role  = "${aws_iam_instance_profile.service_role.arn}"
    spot_iam_fleet_role = "${aws_iam_role.spot_fleet_role.arn}"
    image_id = "${data.aws_ami.base_ami.image_id}"
    max_vcpus = 1000
    min_vcpus = 0
    bid_percentage = 50
    security_group_id = "${aws_security_group.outbound_sg.id}"
    private_subnets = "${join(", ", var.private_subnets)}"
  }
}

I then try to reference the keepers attributes from other resources like this

resource "aws_batch_compute_environment" "compute_env" {
  compute_environment_name = "compute-env-v1-${random_string.rand.result}"
  compute_resources {
    instance_role =  "${random_string.rand.keepers.instance_role}"
    ...
  }
  type = "MANAGED"

  lifecycle {
    create_before_destroy = true
  }
}

However, this is resulting in errors such as

aws_batch_compute_environment.compute_env: Resource 'random_string.rand' does not have attribute 'keepers.instance_role' for variable 'random_string.rand.keepers.instance_role'

What am I doing wrong here? Why does the random_string resource not have these attributes for the keepers values?

Thanks, David