{USER_TT_TRAYS.SCAN_TYPE} <> "S"

There are three possible scan types - S, M and D. It is pulling the M and D records but it is not including any records that are null. Is there a way to include the records that are null?

I have already tried adding

 

or isnull({USER_TT_TRAYS.SCAN_TYPE})

 

and it did not bring me the results I needed.