Return NULL result set

Greetings,

Query
=====
SET @MonthlyInterest := 1000;
SET @IntPaidToMontht := 900;
SET @UP_ActualWDBI := 500;

UPDATE
table1
SET
Interest = IF(@MonthlyInterest - @IntPaidToMonth >=
@UP_ActualWDBI,
@UP_ActualWDBI,
@MonthlyInterest - @IntPaidToMont),

Capital = IF(@MonthlyInterest - @IntPaidToMonth >=
@UP_ActualWDBI,
0,
@UP_ActualWDBI - (@MonthlyInterest -
@IntPaidToMont));


RESULT SET
==========
Interest := NULL
Capital := NULL


What wrong with my query?

I thought I should get 100 for Integer and 400 for
Capital!

Meanwhile, I'm using MySQL 4.1.10 and defined Integer
and Capital as BIGINT UNSIGNED in table1.

Thanks in advance.



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