This letter proposes an efficient null space-based homomorphic message authentication code scheme providing resistance against tag pollution attacks in random linear network coding, where these attacks constitute a severe security threat. In contrast to data pollution attacks, where an adversary injects into the network corrupted packets, in tag pollution attacks the adversary corrupts (i.e. pollutes) tags appended to the end of the coded packets to prevent the destination nodes from decoding correctly. Our results show that the proposed scheme is more efficient compared to other competitive tag pollution immune schemes in terms of computational complexity.