Sql Server deterministic user-defined function

I have the following user-defined function:

create function [dbo].[FullNameLastFirst]
(
    @IsPerson bit,
    @LastName nvarchar(100),
    @FirstName nvarchar(100)
)
returns nvarchar(201)
as
begin
    declare @Result nvarchar(201)
    set @Result = (case when @IsPerson = 0 then @LastName else case when @FirstName = '' then @LastName else (@LastName + ' ' + @FirstName) end end)
    return @Result
end

I can't create an Index on a computed column using this function cause it's not deterministic. Someone could explain why is it not deterministic and eventually how to modify to make it deterministic? Thanks