There is a good conference paper on the topic from CICC 2004
RF distortion analysis with compact MOSFET models
Bendix, P.; Rakers, P.; Wagh, P.; Lemaitre, L.; Grabinski, W.; McAndrew, C.C.; Gu, X.; Gildenblat, G.;
Custom Integrated Circuits Conference, 2004. Proceedings of the IEEE 2004
3-6 Oct. 2004 Page(s):9 - 12

The basic answer is that the equations used to model velocity saturation for short channel devices may not have higher order derivatives at Vds=0V. In some models, this expression may have a singularity at Vds=0.

In the past, some engineers have used a small current source to force a small vds when using a floating fet.