We show that the thermodynamic conjugate of the extra stress is a more suitable independent variable than the extra stress itself for the thermodynamic description of some nonlinear models of viscoelasticity. This brings more precision to the knowledge of non-equilibrium thermodynamic potentials, and provides an illustration of a recently proposed conservation-dissipation formalism which allows bringing extended irreversible thermodynamics to the nonlinear regime. (C) 2016 Elsevier B.V. All rights reserved.