[dba-SQLServer] SQL - What's a null in Reporting Services

Hello all

In Access to test for null and replace it with x I would do something like

Iif(isnull([SomeField]),"NothingThere","SomethingThere")

When I try this in a text box in reporting services it says

Replace Null with System.DBNull

So I did and now get the error...

dBNull is a type in system and cannot be used as an expression

<Sigh>

Anyone know what the correct syntax should be to test for Null and put in my own
value if true and 

Leave the FieldValue if not true?

Many thanks

Darren