Multi-Row Forumal - why isn't this working, does not give error.

Here's what the data I'm trying to act on looks like:

I want to copy down from the row above where the data is [Null]

Here's the forumla I've got in the Multi-Row Formula tool -

There are two examples - neither works or gives an error?

1)

IF [Row+1:T-D-S] == "[Null]" then [T-D-S]
else [T-D-S]
endif

2)

IF isnull([Row+1:T-D-S]) then [T-D-S]
else [T-D-S]
endif

IF it worked then in column T-D-S, in row 2 it would say 'Dublet'

Any ideas why this does not give an error or work? 

What else might I try?