The problem for electromagnetic field becomes clearer when the generalized Poynting theorem is used in a physical system for all active fields. In this case, it is shown that the cause of the problem is the difference between the four-vector and four-tensor. Indeed, the energy and momentum of the system form a four-momentum. However, the energy and momentum densities of electromagnetic field are temporary components of the stress-energy tensor and do not form a four-vector. The same applies to integrals over the volume of these components.