Having trouble with an "IF" expression

This should be a simple issue, but it is kicking my arse. I want to say if last name is NOT blank, print last name, else print organization name. When last name is blank and there is an organization name, nothing prints for the field. What am I doing wrong???

ISNOTHING, (Fields!OrganizationName.Value), (Fields!FirstName.Value &" "& Fields!LastName.Value))