title: Custom resource outputs not found
body: <p>I've written a custom resource in Go using <code>cloudformation-cli-go-plugin</code>, it's failing when I try and use it in a stack with</p>
  <pre><code>Unable to retrieve Guid attribute for MyCo::CloudFormation::Workloads, with error message NotFound guid not found.
  </code></pre>
  <p>The stack:</p>
  <pre><code>AWSTemplateFormatVersion: 2010-09-09
  Description: Sample MyCo Workloads Template
  Resources:
    Resource1:
      Type: 'MyCo::CloudFormation::Workloads'
      Properties:
        APIKey: &quot;&quot;
        AccountID: &quot;&quot;
        Workload: &gt;-
          workload: {entityGuids: &quot;&quot;, name: &quot;CloudFormationTest-Create&quot;}
  Outputs:
    CustomResourceAttribute1:
      Value: !GetAtt  Resource1.Guid
  </code></pre>
  <p>If I remove the <code>Outputs</code> stanza the stack runs successfully and I can see the created resource.</p>
  <p>Running with SAM locally I've verified that <code>Guid</code> is in fact always returned. FWIW the resource passes all of the contract tests, <code>Guid</code> is the <code>primaryIdentifier</code>, and is a <code>readOnlyProperties</code>.</p>
  <p>I've tried several tests playing with the <code>!GetAtt</code> definition, all of which fail with schema errors so it appears the CF is aware of the format of the resource's properties.</p>
  <p>Suggestions and/or pointers would be appreciated.</p>