Malformed input-Template format error: Every Default member must be a string (Cloudformation template problems)

This problem occurs when you specify Parameters and have their default value calculated in some way (usually be referencing other parameters).

This definition is not valid - parameters' default values must be strictly strings that the command line tool can replace with other strings.

I received the same error message when using the Parameter of type CommaDelimitedList in my cfn template.

  LoadBalancerSubnets:
    Description: List of subnets for the ApplicationLoadBalancer
    Type: CommaDelimitedList
    Default: [ "subnet-123456", "subnet-012345" ]

This was due to my mis-interpretation of the CommaDelimitedList type. I thought it is actually a list but it turns out that the value should be a single String value in which the various elements should be separated by a comma. So, I changed my template to look like this:

  LoadBalancerSubnets:
    Description: List of subnets for the ApplicationLoadBalancer
    Type: CommaDelimitedList
    Default: "subnet-123456,subnet-012345"

and this worked.

The error was a very generic one and there may be other scenarios as well in which the same error gets thrown.

However, I thought about sharing my experience with this error so it may help others who got stuck with the same problem.

This happens in Cloudformation Outputs definition when the result of a ´Fn::...´ is a list and not a string.

To make strings out of Lists you can ´Fn::Join´ the list result to a comma separated string. e.g.:

 "VpcSubnets": {
      "Export": {
        "Name": "VpcSubnets"
      },
      "Value": {
        "Fn::Join": [
          ",",
          {
            "Fn::GetAtt": [
              "MyVpc",
              "Subnets"
            ]
          }
        ]
      }
    }

I have seen this error for the Outputs description field as well (I was using !Sub). Had to remove !Sub and just use plain text.

I think you have unnecessary !Ref or !Sub like this one

Parameters:
  RelationTableName: 
    Description: relations table name
    Type: String
    Default: !Ref "relations"