A question about run-time errors when class members are undefined

Hello, We are adding classes and instances to Helium. We wondered about the aspect that it is allowed to have a class instance of which not all fields have a piece of code/value associated with them, and that as a result when you happen to call these, a run-time error results. (see Sec. 4.3.2 of the Haskell 2010 report). Does anyone know of a rationale for this choice, since it seems rather unhaskell-like. best, Jur