Failure associating EIP: InvalidParameterCombination: You must specify an allocation id when mapping an address to a VPC instance

Given this:

resource "aws_instance" "example" {
  ami           = "ami-2757f631"
  instance_type = "t2.micro"
}

resource "aws_eip" "ip" {
  instance = "${aws_instance.example.id}"
}

from https://www.terraform.io/intro/getting-started/dependencies.html

I get an error:

* aws_eip.ip: Failure associating EIP: InvalidParameterCombination: You must specify an allocation id when mapping an address to a VPC instance

Any idea why?