In this paper, we analyze the security of a recent image encryption algorithm based on an improper fractional-order chaotic system suggested by Zhao et al. The fatal flaw in the cryptosystem is that the keystream generated depends on neither the plain-image nor the cipher-image. Another main issue with this algorithm is using the same key (the last key in the keystream) in all encryption equations. Based on these points, it is easy to recover the plain-image and the keystream by applying chosen plaintext attack in only one plain-image. Both mathematical analysis and experimental results confirm the feasibility of this attack. As a result, the cryptosystem under study is not suitable for cryptography.