I'm unable to join floating ip and droplet creation.

# I create a resource:
resource "digitalocean_droplet" "mail" {
  image = "centos-7-x64"
  name = "mail"
  region = "${var.region}"
  size = "2gb"
  private_networking = true
}

#I create a floating ip:
resource "digitalocean_floating_ip" "mail" {
  droplet_id = "${digitalocean_droplet.mail.id}"
  region = "${var.region}"
}

The IDs are the same, but the IP's are not.

$ doctl compute droplet list
ID      Name    Public IPv4 Public IPv6 Memory  VCPUs   Disk    Region  Image           Status  Tags
38296000    mail    188.166.174.104         2048    2   40  lon1    CentOS 7.3.1611 x64 active  
$ doctl compute fip list
IP      Region  Droplet ID  Droplet Name
67.207.68.145   lon1    38296000    mail

How do I get the droplet to take the floating IP address?

Thanks,