Using Group By Clause In Union Query

Hi all,

I have two table having datas like

Table1
--------------------------------------------------------------------
A C1 C2 C3 C4
--------------------------------------------------------------------
x 0 0 3 2
x 0 1 0 2
x 0 0 2 1
y 1 5 2 0


Table2
--------------------------------------------------------------------
A C1 C2 C3 C4
--------------------------------------------------------------------
x 0 0 1 4
y 1 0 3 1
y 1 2 0 0
y 0 0 5 1

select * from(
select A,C1,C2,C3,C4 from Table1 group by A
union
select A,C1,C2,C3,C4 from Table2 group by A
)as t

Result:
--------------------------------------------------------------------
A C1 C2 C3 C4
--------------------------------------------------------------------
x 0 1 5 5
y 1 5 2 0
x 0 0 1 4
y 2 2 8 2

But i need the result like i.e grouped by column 'A'
--------------------------------------------------------------------
A C1 C2 C3 C4
--------------------------------------------------------------------
x 0 1 6 9
y 3 7 10 2

select * from(
select A,C1,C2,C3,C4 from Table1 group by A
union
select A,C1,C2,C3,C4 from Table2 group by A
)as t group by A

The above query gives the following error
[Error Code: 8120, SQL State: S1000] Column 't.C1' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Please help me out.

-Anand