Assigning results for a FOR XML EXPLICIT UNION to a variable

This query works fine

SELECT 1 AS TAG, NULL AS parent, 1 [dog!1]
UNION (SELECT 1,NULL, 3) 
FOR XML EXPLICIT, ROOT('dogs')

I would like to assign it to a variable. This works fine without the union:

DECLARE @x xml
SET @x = ((
  (SELECT 1 AS TAG, NULL AS parent, 1 [dog!1]
   FOR XML EXPLICIT, ROOT('dogs'))
  )) 

As soon as I introduce the union it doesn't work:

DECLARE @x xml
SET @x = ((
  (SELECT 1 AS TAG, NULL AS parent, 1 [dog!1]
   UNION (SELECT 1,NULL, 3) 
   FOR XML EXPLICIT, ROOT('dogs'))
   )) 

Tried all sorts of things. What am I missing

You just need to add one more sub-query level like this:

DECLARE @X XML = 
(
  SELECT * FROM
  (
    SELECT 1 AS TAG, NULL AS parent, 1 [dog!1]
    UNION 
    SELECT 1,NULL, 3
  )X
  FOR XML EXPLICIT, ROOT('dogs')
);

SELECT @X;