nested properties

I'm querying the search API through Postman with the includefields parameter and have a question that I could not find docs for: 

How do I target nested properties? I'd like to return the author displayname from content > createdbyuser > displayname. The following query does not seem to be correct: 

https://mysite/api.ashx/v2/search.json?Category=blog&PageSize=1&includefields=url,content.createdbyuser.displayname 
Returns: 


"PageSize": 1, 
"PageIndex": 0, 
"TotalCount": 75, 
"SearchResults": [ 

"Url": "https://mysite/group/b/blog/posts/post", 
"Content": { 
"Url": "https://mysite/group/b/blog/posts/post


], 
"FieldFacets": [], 
"DateRangeFacets": [], 
"Info": [], 
"Warnings": [], 
"Errors": []