Ansible: register: ec2 / with_items: ec2.instances - results in "str object has no attribute"

I have a playbook that's very similar to examples online:

tasks:
- name: create an EC2 instance
ec2:
[-snip-]
register: ec2
- name: print it
debug: msg="item has - {{ item.public_ip }}"
with_items: ec2.instances

This gives the error:

fatal: [localhost] => One or more undefined variables: 'str object' has no attribute 'public_ip'

If I change it to {{ item }} I see:

TASK: [print it] ********************
ok: [localhost] => (item=ec2.instances) => {
"item": "ec2.instances",
"msg": "item has - ec2.instances"
}

It seems that 'ec2.instances' is merely a string? Lastly, if I use "with_items: ec2.results", I can iterate through it but I have to reference it as item.instances.0.public_ip

Any idea why ec2_instances appears to be a string as opposed to a dictionary?

Edit:
: arp_kiki/pts/55:0.08:new; ansible --version ansible 1.5.4

after upgrading:

ok: [localhost] => (item=ec2.instances) => {
"item": "ec2.instances",
"msg": "item has - ec2.instances"
}

PLAY RECAP **********************
localhost : ok=2 changed=1 unreachable=0 failed=0

: arp_kiki/pts/55:0.31:new; ansible --version
ansible 1.9.1