In the envelope function treatment of quantum wells and superlattices, an effective mass Hamiltonian including corrections to the quadratic dispersion relation is commonly employed to describe non-parabolicity and other complications of the band structure. A careful definition of the boundary conditions used to connect the envelope functions at the interfaces is required to consistently take such higher order effects into account to a given order of approximation. It is possible to develop a single band scheme to describe the light-hole non-parabolicity implicitly accounting for the coupling to other bands. With respect to the conduction electron case, the coupling between the split-off and light-hole bands brings about qualitative changes in the boundary conditions. Model calculations show how the non-parabolicity affects the energy levels not only through the modified (i.e. non-quadratic) dispersion relation, but also through the consistently modified boundary conditions. The present simple theory compares favorably with experimental data and more refined theoretical treatments.