packer ignores ssh_private_ip

Only ignored when associate_public_ip_address is set. i.e. with the following config it can ssh in just fine (but yum doesn't work obviously):
{
  "builders": [
    {
      "type": "amazon-ebs",
      "access_key": "redacted",
      "secret_key": "redacted",
      "region": "eu-west-1",
      "source_ami": "ami-8cff51fb",
      "instance_type": "t2.micro",
      "ssh_username": "ec2-user",
      "ami_name": "packer_puppet {{timestamp}}",
      "ssh_private_ip": true,
      "subnet_id": "subnet-98fafbde",
      "security_group_id": "sg-869e5de3",
      "vpc_id": "vpc-273cd342",
      "ssh_timeout": "5m"
    }
  ],

  "provisioners": [
    {
      "type": "shell",
      "script": "setup_things.sh"
    }
  ]
}

When I change the config to this (attempting to allow yum to do its thing during provisioning):

{
  "builders": [
    {
      "type": "amazon-ebs",
      "access_key": "redacted",
      "secret_key": "redacted",
      "region": "eu-west-1",
      "source_ami": "ami-8cff51fb",
      "instance_type": "t2.micro",
      "ssh_username": "ec2-user",
      "ami_name": "packer_puppet {{timestamp}}",
      "ssh_private_ip": true,
      "subnet_id": "subnet-98fafbde",
      "security_group_id": "sg-869e5de3",
      "vpc_id": "vpc-273cd342",
      "associate_public_ip_address": true,
      "ssh_timeout": "5m"
    }
  ],

  "provisioners": [
    {
      "type": "shell",
      "script": "setup_things.sh"
    }
  ]
}

It fails:

2014/10/31 15:39:00 ui: ==> amazon-ebs: Waiting for instance (i-170c1c55) to become ready...
2014/10/31 15:39:00 packer-builder-amazon-ebs: 2014/10/31 15:39:00 Waiting for state to become: running
2014/10/31 15:39:00 packer-builder-amazon-ebs: 2014/10/31 15:39:00 Allowing 300s to complete (change with AWS_TIMEOUT_SECONDS)
2014/10/31 15:39:17 ui:     amazon-ebs: Public DNS: ec2-54-72-189-72.eu-west-1.compute.amazonaws.com
2014/10/31 15:39:17 ui:     amazon-ebs: Public IP: 54.72.189.72
2014/10/31 15:39:17 ui:     amazon-ebs: Private IP: 10.10.30.12
2014/10/31 15:39:17 ui: ask: ==> amazon-ebs: Pausing after run of step 'StepRunSourceInstance'. Press enter to continue.
2014/10/31 15:40:39 ui: scan err: unexpected newline
2014/10/31 15:40:39 packer-builder-amazon-ebs: 2014/10/31 15:40:39 Waiting for SSH, up to timeout: 5m0s
2014/10/31 15:40:39 ui: ==> amazon-ebs: Waiting for SSH to become available...
2014/10/31 15:40:54 packer-builder-amazon-ebs: 2014/10/31 15:40:54 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:41:14 packer-builder-amazon-ebs: 2014/10/31 15:41:14 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:41:34 packer-builder-amazon-ebs: 2014/10/31 15:41:34 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:41:55 packer-builder-amazon-ebs: 2014/10/31 15:41:55 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:42:15 packer-builder-amazon-ebs: 2014/10/31 15:42:15 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:42:35 packer-builder-amazon-ebs: 2014/10/31 15:42:35 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:42:55 packer-builder-amazon-ebs: 2014/10/31 15:42:55 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:43:15 packer-builder-amazon-ebs: 2014/10/31 15:43:15 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:43:35 packer-builder-amazon-ebs: 2014/10/31 15:43:35 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:43:56 packer-builder-amazon-ebs: 2014/10/31 15:43:56 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:44:16 packer-builder-amazon-ebs: 2014/10/31 15:44:16 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:44:36 packer-builder-amazon-ebs: 2014/10/31 15:44:36 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:44:56 packer-builder-amazon-ebs: 2014/10/31 15:44:56 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:45:16 packer-builder-amazon-ebs: 2014/10/31 15:45:16 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:45:37 packer-builder-amazon-ebs: 2014/10/31 15:45:37 TCP connection to SSH ip/port failed: dial tcp 54.72.189.72:22: i/o timeout
2014/10/31 15:45:39 ui error: ==> amazon-ebs: Timeout waiting for SSH.
2014/10/31 15:45:39 packer-builder-amazon-ebs: 2014/10/31 15:45:39 SSH wait cancelled. Exiting loop.
2014/10/31 15:45:39 ui: ask: ==> amazon-ebs: Pausing before cleanup of step 'StepRunSourceInstance'. Press enter to continue.