SPICE doesn't like zero Ohm resistors, which is why I imagine it causes problems. It also somewhat mirrors reality, since the potentiometer will always have a tiny wiper resistance.
As mentioned above, there's no need to simulate this, just work it out. In reality, you'll probably require some out of range operation, due to component tolerances. Potentiometers typically have a very poor tolerance of 10% or worse. One way round this is to connect another, much lower value 1% tolerance resistor in parallel with the potentiometer, but it will result in extra power draw.