The dispersion characteristics of whistler waves propagating along a longitudinally magnetised and lossy plasma slab are numerically calculated. It is shown that, if the wavelength of the whistler waves is much smaller than the transverse dimensions of the plasma slab, a nonzero collision frequency can have a large influence on the dispersion characteristics; for such small wavelengths one should expect the whistler dispersion to be close to that of the corresponding plane waves, but this is, for certain parameter values, true only if the collision frequency is assumed to be nonzero.