The Nakamoto longest chain protocol has served Bitcoin well in its decade long existence. It is remarkably simple and uses only basic cryptographic primitives, but its proof-of-work framework is energy wasting. Proof-of-stake (PoS) protocols are an energy efficient alternative; however they are significantly complicated and promise weaker security guarantees. An effort to mimic the Nakamoto protocol directly in the PoS setting is made in [10, 11] with security shown only for a class of purely private attacks. In this paper we demonstrate a new, and fatal, attack on the protocol of [10, 11]. This attack motivates the design of a new family of Nakamoto-style longest chain PoS protocols, with a formal proof of their security against all possible attacks in a general security model.