modifying a dictionary in a loop

bad.yml

   # this fails on the "set" tag
   - set_fact: instances="{%- set instances = dict() %}
                           {%- for group in group_names %}
                               {%- for host in groups[group] %}
                                   {%- if hostvars[host].get('InstanceId', None) %}
                                      {%- if hostvars[host].get('Platform', None) == 'windows' %}
                                          {%- set tmp = instances.setdefault(host, hostvars[host]['InstanceId']) %}
                                      {%- endif %}
                                   {%- endif %}
                                {%- endfor %}
                           {%- endfor %}{{instances}}"