SQL Server requires CAST when using NULL in anchor part of recursive CTE

with a (intkey, stringcolumn, lvl) as
    (
    select intkey, NULL as stringcolumn, 0 as lvl from bqt1.smallb where intkey = 1
    union all 
    select n.intkey, n.stringkey as stringcolumn, rcte.lvl + 1 as lvl from bqt1.smallb n inner join a rcte on n.intkey = rcte.intkey + 1
    )
 select * from a

WITH a (intkey, stringcolumn, lvl) AS
    (
    SELECT SmallB.IntKey, NULL AS stringcolumn, 0 AS lvl FROM SmallB WHERE SmallB.IntKey = 1
    UNION ALL
    SELECT n.IntKey, n.StringKey AS stringcolumn, (rcte.lvl + 1) AS lvl FROM SmallB n INNER JOIN a rcte ON n.IntKey = (rcte.intkey + 1)
    )
SELECT a.intkey, a.stringcolumn, a.lvl FROM a

WITH a (intkey, stringcolumn, lvl) AS
    (
    SELECT SmallB.IntKey, CAST(NULL AS VARCHAR(10)) AS stringcolumn, 0 AS lvl FROM SmallB WHERE SmallB.IntKey = 1
    UNION ALL
    SELECT n.IntKey, n.StringKey AS stringcolumn, (rcte.lvl + 1) AS lvl FROM SmallB n INNER JOIN a rcte ON n.IntKey = (rcte.intkey + 1)
    )
SELECT a.intkey, a.stringcolumn, a.lvl FROM a