❔ Optional function parameter

I have this function

public void abc(Entity entity, List<string> directions, Position occupied = null

public void abc(Entity entity, List<string> directions, Position occupied = null

but I get

CS1750: A type of value <null> cannot be used as a default parameter because there are no standard conversions to type Position

CS1750: A type of value <null> cannot be used as a default parameter because there are no standard conversions to type Position