Use recordset and return Invalid string or buffer length when the value is '' (not null)

There is a simple schema "test", and a table "testtable", and there is only column "testc" for char(10).

There is only one row in test.testtable, and the value is '' (not null)

when the asp use recordset to retrive the row for test,testtable,like this
sql = "select testc from test.testtable"
ds = conn.execute(sql)

it always return
[ma-2.0.11][mariadb-10.1.16] Invalid string or buffer length

I need to change the sql statement to
sql = "select (case testc when '' then NULL else testc end) as testc from test.testtable"
ds = conn.execute(sql)

It can be worked, why doesnt return '' for this case?

Thanks.