Terraform + VMWare – can’t provision a server

I'm trying to do some really basic stuff with Terraform on VMWare. I'm pretty sure I must be doing something obvious wrong as this is a really simple use case.

I have a test.tf file like this:

# Configure the VMware vSphere Provider
provider "vsphere" {
  user           = "${var.vsphere_user}"
  password       = "${var.vsphere_password}"
  vsphere_server = "${var.vsphere_server}"

  # if you have a self-signed cert
  allow_unverified_ssl = true
}

# Create a folder
resource "vsphere_folder" "test_folder" {
  path = "test_folder"
  datacenter = "Datacenter"
}


# Create a virtual machine within the folder
resource "vsphere_virtual_machine" "web" {
  datacenter = "Datacenter"
  name   = "terraform-web"
  folder = "${vsphere_folder.test_folder.path}"
  vcpu   = 2
  memory = 4096

  network_interface {
    label = "VM Network"
  }

  disk {
    datastore = "datastore1"
    template = "my-template/my-template.vmdk"
  }
}

I have a variable file with the user, password and VMware server in.

When I run terraform plan, it executes cleanly.

When I run terraform apply, I get:

* vsphere_virtual_machine.web: vm 'my-template/my-template.vmdk' not found

I've tried leaving off the my-template.vmdk off (so pointing at the template directory)
I've tried pointing it at the vmx file

What should I be putting for the disk location? Does anyone have a working example please?