IS NULL seems to return false, even when parameter is NULL

I have a strange problem with the following condition in a SELECT:
AND (("TableData" = "inDate") OR ("inDate" IS NULL))

it works perfectly when the input date in the function (inDate) matchs
a date in the table, but it does not work when the parameter inDate is
NULL.
I recall the function with pgadmin writing NULL as paramater.
It seems that ("inDate" IS NULL) never returns true, even when the
parameter is null...
What's wrong?

Lucazeo