Certificateless cryptography eliminates the need of certificates in public key cryptosystems and solves the inherent key escrow problem in identity-based cryptosystems. In this paper we discuss the existence of short certificateless signature schemes provably secure in formal security models recently established, whose resulting signature consists of a single element of the underlying group by demonstrating key replacement attacks on Du-Wen's and Tso et al.'s short certificateless signature schemes.