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

Question

I'm attempting to validate a cloudformation template, and cfn-validate returns a useless error of

"Malformed input-Template format error: Every Default member must be a string".

Does anybody have an idea on how to debug this? It would be awesome if cfn- validate would return errors or at least resource names when errors are given.

CloudformationTemplates are incredibly painful to debug.

Answer 1

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

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

Answer 2

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.

Answer 3

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.