IsNull expression

[Dhyan]

Whats wrong with this expression, logically i can print any default variable if a field comes as Null. 
= IsNull(Fields.ROE_wo_Reserves, Fields.Flat_Line_Rent) 

I am getting blank value as output everytime. 

[Squall]

Are the fields really null or they are blank? You can try to add a check for empty string:

= IIF(Fields.ROE_wo_Reserves is Null OR Fields.ROE_wo_Reserves = "", Fields.Flat_Line_Rent, Fields.ROE_wo_Reserves)