CASE Does Not Give Correct Result

How do I automatically assign a new cardcode-number? (according to the following formula: highest existing number + 1)

Scenario:
-There are two types of business partners: Customers and Suppliers.
-Customers have the value 'C' in the colomn CardType.
-Suppliers have the value 'S' in the colomn CardType.
-Customers have the following syntax 'C123456' in the colomn CardCode.
-Suppliers have the following syntax 'S123456' in the colomn CardCode.
-Existing CardCode-values in the DB for the Customers: C000001 - C100599.
-Existing CardCode-values in the DB for the Suppliers: S000001 - S200199.

The idea is that when a user creates a new business partner, the CardCode should be automatically filled when a new assigned number (highest existing number + 1), according to the value that is selected in CardType (either the letter 'C' or 'S').


What's been done so far:
SELECT top 1
(CASE
WHEN CardType='C' THEN (SELECT top 1 'C' + '' + cast((substring(T0.CardCode, 2, 7) + 1) as varchar) as [nummer]
FROM OCRD T0
WHERE T0. CardCode like 'C%' AND T0. CardType='C'
order BY T0.CardCode desc FOR BROWSE)
WHEN CardType='S' THEN (SELECT top 1 'S' + '' + cast((substring(T0.CardCode, 2, 7) + 1) as varchar) as [nummer]
FROM OCRD T0
WHERE T0. CardCode like 'S%' AND T0. CardType='S'
order BY T0.CardCode desc FOR BROWSE)
END)
FROM OCRD T0


The current result:
The result that it gives is 'C100600'.
The problem however is that it always gives this result and does not take into account what has been selected in CardType.

When I add the following: "order BY T0.CardCode desc FOR BROWSE" it gives the result 'S200200'.

So, what does work is that it takes the highest existing value and adds 1. But what doesn't work is the taking account what value is selected in CardType.

Does anyone know how I can make this work?