Blank Vs NULl Parameter

I am creating a cascading report , The 1st parameter Values here are NULL as well as ''However , when i write query for 2nd parameters I pass the values as where (Program_group = @Master OR @Master IS NULL OR @Master = ' ')ABut The list I get in both cases be it blank or with Null is the same. Somehow its not treating NULL as NULL its treating it as blank too.P.S : I have selected Allow Null and Allow Blank in the parameter properties.Anything that I am missing????????????