SyntaxError when trying to order by child

Using an order by where I'd like to sort on a child will result in a syntax error.

For example:
?$orderby=Rating,Category/Name desc

Will result in:

{ [SyntaxError: Expected " ", "&", ",", "." or [a-zA-Z0-9\-_] but "/" found.]
  name: 'SyntaxError',
  expected: [ '" "', '"&"', '","', '"."', '[a-zA-Z0-9\\-_]' ],
  found: '/',
  message: 'Expected " ", "&", ",", "." or [a-zA-Z0-9\\-_] but "/" found.',
  offset: 33,
  line: 1,
  column: 34 }

It's however specified as being valid according to these docs:
http://www.odata.org/documentation/odata-version-2-0/uri-conventions/