I have problem in using the following SQL in IBO

I have a table with structure
F1 F2 Q1
1 1 10
1 2 20
1 3 40
2 1 80
2 2 160
(F1,F2) is the primary key of the table

and would like to get the result table with accumulated quantity Q2 of
Q1 as below:
F1 F2 Q1 Q2
1 1 10 10
1 2 20 30
1 3 40 70
2 1 80 80
2 2 160 240

So I use following SQL
select F1, F2, Q1,
(select sum(Q1) from MyTable where F1 = a.F1
and F2 <= a.F2) as Q2
from MyTable a
order by F1, F2

I was using with IB_SQL.exe (version 4.2Ha) in win98, Firebird 1.0 for
testing.

But I got the following errors message:
ISC ERROR CODE:335544569

ISC ERROR MESSAGE:
Dynamic SQL Error
SQL error code = -502
Declared cursor already exists

STATEMENT:
TIB_Query: "<TApplication>.frmSQL.frmQuery.qrQuery."

And SqL message:

SQL ERROR CODE:-502

SQL ERROR MESSAGE:
The cursor identified in an OPEN statement is already open.



The same SQL was acceptable when I using the other tools such as
ibadmin. I was quite confused for what mistake I might take.

Please advise if anyone encounter similiar problem and the ways that
I could try to solve the problem.

Thanks for your kind attention.