SQL(Interbase) : Case of string with Aggregate function

Question

I am not able to make the following SQL work in Interbase: i get the following error:

Error at line 1 Attempt to execute an unprepared dynamic SQL statement

SQL:

SELECT CASE
   WHEN kl='K' THEN (SUM(DB)-SUM(CR))
   WHEN kl='L' THEN (SUM(CR)-SUM(DB))
   END AS Saldo
FROM  Table1
GROUP BY KL

for the Table1 defined this way:

Column name   Data type
------------  ------------
KL            varchar(1)
DB            int
CR            int

Yet it works in SQL fiddle

the following also works!

SELECT CASE 
   WHEN kl='K' THEN 1
   WHEN kl='L' THEN 2
   END as Saldo
FROM  Table1
Group By KL

Answer 1

Found It!

SELECT Sum(CASE
       WHEN kl='K' THEN db
       WHEN kl='L' THEN cr
       END )
      -Sum(CASE
       WHEN kl='K' THEN CR
       WHEN kl='L' THEN DB
       END )

 FROM  Table1
Group By KL