Column alias in UNION with GROUP BY query

Simplified sample query
SELECT greet FROM (SELECT 'hi' AS greet
UNION SELECT 'hello'
FROM (SELECT '') AS _ GROUP BY NULL) AS tbl WHERE greet != ''

Actual result
[mysqli: 1054] Unknown column 'greet' in 'order clause'

Expected result
Query passes (returns 'hihello')

Additional info
The following changes each make the query pass:

• Change line 2 to UNION SELECT 'hello' AS greet
• Remove the GROUP BY NULL clause