DigitalOcean Terraform can't work with Count and droplet_ids

When trying to run the following script I get an error and I don’t understand why. All the example show it should be done this way:

resource "digitalocean_loadbalancer" "web" {
  name   = "loadbalancer-1"
  region = "ams3"

  forwarding_rule {
    entry_port     = 80
    entry_protocol = "http"

    target_port     = 80
    target_protocol = "http"
  }

  healthcheck {
    port     = 80
    protocol = "tcp"
  }

  droplet_ids = ["${digitalocean_droplet.web.*.id}"]
}

resource "digitalocean_droplet" "web" {
  count              = 2
  image              = "ubuntu-18-04-x64"
  name               = "web-${count.index + 1}"
  region             = "ams3"
  size               = "s-1vcpu-1gb"
  private_networking = true
  user_data          = "${file("config/webuserdata.sh")}"
  ssh_keys = [
    "${var.ssh_fingerprint}"
  ]
}

The error:

Error: Incorrect attribute value type

  on web.tf line 18, in resource "digitalocean_loadbalancer" "web":
  18:   droplet_ids = ["${digitalocean_droplet.web.*.id}"]

Inappropriate value for attribute "droplet_ids": element 0: number required.

I’m new to terraform and I don’t know how to solve this issue.