Petstore Swagger API - Cancelled a request due to invalid content

Update:  I've tried to post this to the SoapUI Pro board, but I can't select it from the drop down list.

I'm trying to test the Swagger API as demonstrated in the SoapUI 201 webinar. 

I've followed the steps in the video:

 -  import Swagger: http://petstore.swagger.io/v2/swagger.json

 -  create Test Case (Add Pet) & Test Step

  - edit request (add id and name - same as in video)

When I submit the request I receive the following error:  "Cancelled a request due to invalid content".  (see screenshot)

However, if I change to the 'Form' or 'Outline' view (see screenshots), the request is sent and a response is successfully received.

Can anyone explain why this is?  Is it my setup in ReadyAPI, or an issue with the Swagger API?

Note:

I've seen another post that recommends changing preferences to disable 'Always validate request messages before they are sent."   I've tried this and it works, HOWEVER it doesn't seem right to turn this off.  I'd rather understand WHY the message is failing validation.