Why do I get a data conversion error when merging an array (where update works fine)?

// Data conversion error converting 
// "'[5, 6]' (B: ""ISET"" INTEGER ARRAY)"
MERGE INTO B
USING VALUES (?) _v(arr)
 ON B.id IN(1)
    WHEN MATCHED THEN
      UPDATE SET arr = _v.arr
    WHEN NOT MATCHED THEN
      INSERT (arr) VALUES (_v.arr)

// works
UPDATE B
SET arr = ?
WHERE B.id IN(1)

It seems like the array is received like a text string for the merge although I prepare the array the same way for both update and merge:

preparedStatement.setArray(1, connection.createArrayOf("INT", Array(5, 6))

Do I need to add a cast somewhere for arrays or what am I doing wrong?