Unable to look up dict by key if the dict key contains a variable

[Aneesh Joseph]

vars.yml

---

volumes:

 "vol-1-{{env}}":

   size: 500

   fs: "ext3"

play.yml

- hosts: localhost
  connection: local
  gather_facts: no
  vars_files:
   - vars.yml
  tasks:
  - set_fact:
     volume_name: "vol-1-{{env}}"
  - debug: msg='{{ volumes[volume_name].size }}'

ansible-playbook test.yml -e "env=dev1"

 I would expect the above play to output 500, but it fails instead with the below error. Is this the expected behaviour for a dict with variables in the key field? 

 The error was: 'dict object' has no attribute u'vol-1-fut1'