How do I set the value of a derived column conditionally?

I created a new column in a derived column transformation.  The expression is below.

@[User::mod] == "AA" || @[User::mod] == "SA" ? "01"

which I think simply states if my variable is "AA" or "SA"  set the new column value to "01"  I'm getting the following

I want to be able to set the value to string "01" if the condition is true.  I also want to add and esle set it to spaces.  I don't want null in my derived column.

Thanks for any and all help,

sharon

shash