title: AWS specific parameters and EC2 SecurityGroupIds List String Error
body: <p>I have a rather annoying issue which I am unable to resolve and will do my best to explain.</p>
  
  <p>The following cut down example works in which I am able to reference a parameter and assign the security groups to my instance via the SecurityGroupIds property:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>"Parameters" : {
        "pDefaultSg" : {
          "Description" : "AWS2 VPC default security groups",
          "Type" : "List&lt;AWS::EC2::SecurityGroup::Id&gt;",
          "Default" : "sg-245xxxxx,sg-275xxxxx,sg-235xxxxx" 
        }
      }
  
      "Resources" : {
        "ec2Instance" : {
          "Type" : "AWS::EC2::Instance",
          "Properties" : {
          "SecurityGroupIds" : { "Ref" : "pDefaultSg" } 
        }
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>The issue begins when I also want to add a second value to the SecurityGroupIds property referencing a security group resource instantiated within the same template:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>"Resources" : {
      "ec2Instance" : { ...
          "SecurityGroupIds" : [ { "Ref" : "pDefaultSg" }, { "Fn::GetAtt" : "sgDb", "GroupId" } ],
      ....  
  
      "sgDb" : {
          "Type" : "AWS::EC2::SecurityGroup",
          "Properties" : { ...</code></pre>
  </div>
  </div>
  </p>
  
  <p>I am then unable to avoid the following error causing the Cloudformation stack to rollback:</p>
  
  <p>Value of property SecurityGroupIds must be of type List of String</p>
  
  <p>I would really appreciate any pointers.</p>
  
  <p>Many Thanks</p>