Fetch returns more than requested number of rows

Hi folks,

I'm passing a single field from a single row into a variable in a procedure. The following example works if MSEG query results in a single row;

EXAMPLE 1: lets say I have a one row in MSEG table;

Row1 MENGE: 100

select a.MENGE into OnHandQty from "MSEG" where condition = X

Result is OnHandQty = 100 << SUCCESS!!

EXAMPLE 2: lets say I have two rows in mseg table; 

Row1 MENGE 100

Row2 MENGE  50

To handle multiple rows in the query I thought the following would work;

select sum(a.MENGE) into OnHandQty from "MSEG" where condition = X

I get error 'fetch returns more than requested number of rows'.  Although if I run the SQL without attempting to insert into a variable I can clearly see the result is 1 single row result with value of 150.

Is aggregation not supported when using INSERT INTO VARIABLE in a procedure?

Thanks,

-Patrick