Data conversion required (SQL 7919)

Hi!
I'm writing a embedded SQL RPGIV program and I've stuck on
trying to make my queries more efficient.

C                   Eval      SumQty  =   'Select      '+
C                                         'sum(Qty)    '+
C                                         'from        '+
C                                         'Q           '+
C                                         'Where       '+
C                                         'Q.QKey = 5   '

Qty field is declared in the Q database like this (9,0 packed):
Qty         50    54   P       5    9  0  Quantity

Now to my confusion
What I've understood is that when using SUM function in SQL
one should have the same data type on the result variable as
the declared elements. Fine, did that above but I still get
this 'error' in joblog...

Why do I still get the Data Conversion 'error'? The program
works fine but the conversion bugs me since a millisec is wasted...

I know that I can do a workaround by doing the adding in RPG with
a Do-loop but is that that more efficient than using Sql Sum?