Following is the parameter block :

  parameters :   
    instance_ip_list:
    type: comma_delimited_list
    description: List of floating IPs to be assigned to Instances

Thereafter in the resources block, am accessing it :

 inst_floating_ip:
      type: OS::Neutron::FloatingIP
      properties:
        floating_network: { get_param: public_net }
        floating_ip_address:  { "Fn::Select" : [ { get_param: public_ip_index}, { get_param: [public_ip] } ] }
        port_id: { get_resource: inst_port }

And I supply the parameters in a separate environment file :

 parameter_defaults:
   instance_ip_list: "10.10.149.74,10.10.149.75"

However, on instantiating the stack, the instance_ip_list is empty leading to failure in stack creation. But If I add the following defaults in the template, then it works fine.

default: ["10.10.149.72","10.10.149.73"]

Please can someone help in clarifying why this difference in behaviour.