The demands for high spectral efficiency and massive connections led the researchers to investigate new multiple access techniques for the future wireless networks. Non-Orthogonal Multiples Access (NOMA) is one of them. Hence, the integration of NOMA technique with the other physical layer techniques, such as MIMO, cooperative communication, and cognitive radio, has recently taken considerable attention. Whereas these numerous studies are mostly devoted to reveal the overall capacity and outage performances of NOMA involved systems, the error performances have not been investigated in the literature. In this letter, we analyze error performance of cooperative-NOMA, and the exact end-to-end bit error probability is derived in the closed-form. Then, the derived expressions are validated via simulations. Finally, the effect of the power allocation coefficient on the error performance is discussed.