Unable to interpolate values in Serverless Framework for AWS - aws-lambda

Question

I'm trying to organize this serverless YML file, but getting failures.I've scanned their docs for understanding how to interpolate via file substitution and I just can't figure it out:
I've read all these:
https://www.serverless.com/framework/docs/providers/aws/guide/variables#reference-properties-in-other-files
https://forum.serverless.com/t/split-up-include-reference-serverless-yml-file/3747
https://github.com/AnomalyInnovations/serverless-stack-demo-api/blob/master/serverless.yml#L128-L137
ERRORS
Serverless Error ----------------------------------------
The CloudFormation template is invalid: [/Resources/IamRoleLambdaExecution/Type/Policies/0/PolicyDocument/Statement/2] 'null' values are not allowed in templates
Serverless.yaml
service: newsletter
frameworkVersion: '2'
plugins:
- serverless-bundle
provider:
name: aws
runtime: nodejs12.x
memorySize: 256
stage: ${opt:stage, 'dev'}
region: us-west-2
iamRoleStatements:
- ${file(resources/iam/UsersSubscriptionTable.yml):UsersSubscriptionTableIAM}
resources:
- ${file(/resources/dynamo/UsersSubscriptionTable.yml):UsersSubscriptionTable}
functions:
createEmailEntry:
handler: src/Email.addUser
events:
- http:
method: POST
path: /subscribe
removeEmailEntry:
handler: src/Email.removeUser
events:
- http:
method: GET
path: /unsubscribe
# Not recommended for production-use
custom:
bundle:
linting: false
resources/iam/UsersSubscriptionTable.yml
Resources:
UsersSubscriptionTableIAM:
Effect: Allow
Action:
- dynamodb:PutItem
- dynamodb:GetItem
Resource:
- arn:aws:dynamodb:${aws:region}:${aws:accountId}:table/MyCoolTable
resources/dynamo/UsersSubscriptionTable.yml
Resources:
UsersSubscriptionTable:
Type: AWS::DynamoDB::Table
Properties:
TableName: MyCoolTable
BillingMode: PAY_PER_REQUEST
AttributeDefinitions:
- AttributeName: email
AttributeType: S
KeySchema:
- AttributeName: email
KeyType: HASH

Answer 1

This was a huge pain to figure out:
provider:
name: aws
runtime: nodejs12.x
memorySize: 256
stage: ${opt:stage, 'dev'}
region: us-west-2
iam:
role:
statements:
- ${file(resources/iam/UsersSubscriptionTable.yml):UsersSubscriptionTableIAM}
resources:
Resources:
UsersSubscriptionTable: ${file(resources/dynamo/UsersSubscriptionTable.yml):UsersSubscriptionTable}
Resources shouldn't be an array in this case.
In addition, the iam role statement shouldn't have a resources top-level item in the yml either:
UsersSubscriptionTableIAM:
Effect: Allow
Action:
- dynamodb:PutItem
- dynamodb:GetItem
Resource:
- arn:aws:dynamodb:${aws:region}:${aws:accountId}:table/MyCoolTable
UsersSubscriptionTable:
Type: AWS::DynamoDB::Table
Properties:
TableName: MyCoolTable
BillingMode: PAY_PER_REQUEST
AttributeDefinitions:
- AttributeName: email
AttributeType: S
# Specifies the email as the partition key (primary key)
KeySchema:
- AttributeName: email
KeyType: HASH