Comment 7 for bug 1316842

Lars,

Note: http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/parameters-section-structure.html

See the Default value, it's a string not a list:

"Parameters" : {
  "DbSubnetIpBlocks" : {
    "Description" : "Comma-delimited list of three IP blocks",
    "Type" : "CommaDelimitedList",
    "Default" : "10.0.48.0/24, 10.0.112.0/24, 10.0.176.0/24"
  }
}