title: In aws cloudformation, it giving error when I try to use !sub and !join function together
body: <p>Hi I am trying following in cfn template :</p>
  <pre><code>ManagedPolicyArns:
          - !Ref kinesiss3logpolicy
        PermissionsBoundary: !Sub 
            - !Join [ '',[ 'arn:aws:iam::${AWS::AccountId}:policy/', !Ref AdminPolicy ] ]
  </code></pre>
  <p>where AdminPolicy is a parameter.</p>
  <p>I get following error :</p>
  <pre><code>*[error]ValidationError: Template error: One or more Fn::Sub intrinsic functions don't specify expected arguments. Specify a string as first argument, and an optional second argument to specify a mapping of values to replace in the string*
  </code></pre>
  <p>can someone please help me fix this?</p>