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Safety cases are used to argue that safety-critical systems satisfy the requirements that are determined to mitigate the potential hazards in the systems operating environment. Although typically a manual task, safety cases have been successfully created for systems without many configuration options. However, in highly configurable systems, typically developed as a Product Line (PL), arguing about each possible configuration, and ensuring the completeness of the safety case are still open research problems. This paper presents a novel and general approach, based on Contract-Based Specification (CBS), for the construction of a safety case for an arbitrary PL. Starting from a general CBS framework, we present a PL extensions that allows expressing configurable systems and preserves the properties of the original CBS framework. Then, we define the transformation from arbitrary PL models, created using extended CBS framework, to a safety case argumentation-structure, expressed using the Goal Structuring Notation. Finally, the approach is exemplified on a simplified, but real, and currently produced system by Scania CV AB.