Non-orthogonal multiple access (NOMA) has attracted great recent attention due to its high spectral efficiency. Besides, NOMA can be easily implemented in physical layer, thus its interplays with other psychical layer techniques have been analyzed widely by researches. Cooperative NOMA schemes are the most investigated among these interplays. However, these studies mostly analyze cooperative NOMA schemes in terms of achievable rate and outage probability whereas bit error probability (BEP) for those systems has not been studied well although it is one of the most important performance metrics. In this paper, we investigate the error performance of cooperative NOMA schemes where a decode-forward relay helps NOMA users. We derive exact BEP expressions in closed-form over Nakagami-m fading channels. All derived expressions are validated via computer simulations.