Where is mistake in my procedure?

Please Help!
What is wrong with this procedure!
Why it doesn't sum the fields Obshta_suma and Pay
Why it doesn't return @Obshta_suma and @PayCREATE PROCEDURE dbo.Nalichni

@Date1 varchar(12),
@Date2 varchar(12),
@Obshta_Suma decimal output,
@Pay decimal output,
@prmO_PKO bit output,--boolean 0-false, 1-true
@prmO_RKO bit output--boolean 0-false, 1-true

AS
set @prmO_PKO = 0
set @prmO_RKO=0
if exists(Select Obshta_suma from PKO where Obshta_suma<>Null and (Vnos_Data >= @Date1) and (Vnos_Data <=@Date2))
begin
set @prmO_PKO=1
Select @Obshta_Suma=sum(Obshta_Suma) from PKO where (Vnos_Data >= @Date1) and (Vnos_Data <=@Date2)
end

if exists(Select Data,Pay from RKO where Pay<>Null and ( Data >= @Date1 and Data <=@Date2))
begin
set @prmO_RKO=1
Select @Pay= sum(Pay) from RKO where( Data >= @Date1) and( Data <=@Date2)
end
GO

you cannot say "where foo <> null"

you have to say "where foo is not null"