title: AWS IAM Cloudformation YAML template errror: 'null' values are not allowed
body: <p>I am working on a Cloudformation template for an IAM role that grants cross account read only access. It uses a managed policy for Readonly access as well. So far, I've resolved several errors, but now I'm getting a "'null' values are not allowed in templates" error when I try to validate the template. I think it's a space or syntax thing, but I cannot be sure as it's my first time creating a cloudformation template from scratch and using YAML.</p>
  
  <pre><code>AWSTemplateFormatVersion: '2010-09-09'
  Description: AWS CloudFormation template IAM Role for New Relic to have read access to AWS account
  Resources:
    NewRelicInfrastructure-IntegrationsRole:
      Type: AWS::IAM::Role
      Properties:
        AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
          Effect: Allow
          Principal:
            AWS: 11111111
          Action: sts:AssumeRole
          Condition:
            StringEquals:
            sts:ExternalId: '11111'
    Path: '/'
    ManagedPolicyArns: arn:aws:iam::aws:policy/ReadOnlyAccess
    RoleName: NewRelicInfrastructure-Integrations2
  </code></pre>