Explicit column name error in CTE of UNION

WITH cte (col1) AS

(

  SELECT 1

  UNION

  SELECT 2

)

SELECT col1 FROM cte;

ERROR 1054 (42S22): Unknown column 'col1' in 'field list'

WITH cte (col1) AS

(

  SELECT 1

)

SELECT col1 FROM cte;

+------+

| col1 |

+------+

|    1 |

+------+