Choose returns null

SELECT
    r.[value] as KnownRandomNumber,
    CHOOSE(r.[value], 5, 15, 16, 17) AS KnownRandomResult,
    CHOOSE(FLOOR(RAND() * 4) + 1, 5, 15, 16, 17) AS UnknownRandomResult
FROM
    (SELECT FLOOR(RAND() * 4) + 1 AS [value]) r

Why does the CHOOSE(FLOOR(RAND() * 4) + 1, 5, 15, 16, 17) sometimes return null? Even though using the random number obtained from subquery never returns null?

I'm running this on Microsoft SQL Server in SQL Server Management Studio.