Delta-oriented programming (DOP) provides a technique for implementing Software Product Lines based on modifications (add, remove, modify) to a core program. Unfortunately, such modifications can introduce errors into a program, especially when type signatures of classes are modified in a non-monotonic fashion. To deal with this problem we present a type system for delta-oriented programs based on row polymorphism. This exercise elucidates the close correspondence between delta-oriented programs and row polymorphism.