aws cloudformation - Error Message if IAM::Group Policy given string is Unclear

[colinbjohnson]

Description of Issue

In CloudFormation, if passing a string to the policy property of a IAM::Group resource the following error is given: list index out of range.

Steps to Reproduce

Using aws cloudformation provide a template file that contains the following:
"GroupDevelopers" : { "Type": "AWS::IAM::Group", "Properties": { "Policies": {"Ref": "PolicyDevelopers"} } }

Run

aws cloudformation create-stack --stack-name Test --parameters --template-body file://test.template

Result

aws cloudformation create-stack --stack-name Test --parameters --template-body file://test.template
list index out of range

Expected Result

A more descriptive error message.

Documentation Detailing only Lists Expected

http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-iam-group.html

[jamesls]

Agreed, this is not a good error message. The actual problem is related to when a parameter is of type list:

       --parameters (list)
          A list of Parameter structures that specify input parameters for the
          stack.

which expects space separated values, but if no value is provided, you'll get a bad error message, as shown in your above command:

...  create-stack --stack-name Test --parameters      --template-body file://test.template
                                                ^^^^missing values

You can either:

a) omit the parameter
b) if you meant to explicitly send an empty list to the service (for some services an empty list is significant and means something different than omitting the param altogether), you can use the --parameters [] syntax