Multiple selects from parametrized CTE fails with syntax error

The following query works fine:

PREPARE stmt FROM

'WITH t1 AS (SELECT 1)

SELECT * FROM t1

 UNION ALL

SELECT * FROM t1'

When extracting the "1" into a parameter the query fails with error 1064:

PREPARE stmt FROM

'WITH t1 AS (SELECT ?)

SELECT * FROM t1

 UNION ALL

SELECT * FROM t1'

Tested on OSX 10.13.6. MySQL 8 seems to handle this without trouble.