Problem with sp_executesql

I try to write query that use sp_executesql to query data by Like operation with 1 parameter like below:
execute sp_executesql N'SELECT DISTINCT au_id,
au_lname,au_fname
FROM authors
WHERE au_lname LIKE @.au_lname
',
N'@.au_lname nVarChar',
@.au_lname = N'%Cas%'

but It return all rows regardless of changing condition to any value.

But if i don't use sp_executesql like below:

SELECT DISTINCT au_id,
au_lname,au_fname
FROM authors
WHERE au_lname LIKE N'%Cas%'

It's correct!

Can anyone tell me why?

ThanksChange your code as follows:

N'@.au_lname nVarChar', -->>> N'@.au_lname nVarChar(5)',|||Thank you very much for feedback!