Update inner join on .... And on

[firefly2k8]

Here is the logic:

if field-a and field-b are in table-y then field-c is set to alpha

here is my SQL

UPDATE TableX
INNER JOIN TableY
ON (TableX.FieldA = TableY.Currency AND TableX.FieldB=TableY.Currency)
SET TableX.FieldC = 'alpha';

But this doesn't update when it should. Where am I going wrong