Question: Error, (in simpl/abs) abs is not differentiable at 0

In an effort to draw the unit normal of a vector, n, to a curve defined by a radius curve, r, the following command is inputted to Maple but recieves the error also noted below.

arrow([0,0,0],eval(convert(n,list),t=0*Pi),shape=cylindrical_arrow,color=black);

Error, (in simpl/abs) abs is not differentiable at 0

 However a plot of the components of the unit vector shows that it is well behaved at 0, i.e. it has values of <-1,0,0>

Execution of the following commands demonstrates this:

eval(convert(n,list),t=-0.00000001);

[-1.000000000,1.850000001 10,4.5000000002 10]

eval(convert(n,list),t=0.00000001);

[-1.000000000,-1.850000001 10,-4.5000000002 10]

Being new to MAPLE and computer algebra how does one work around this? Clearly here the function is differntiable, but I am being stopped in my tracks.  What about the situation where there is a dicontiniuty, how do we proceed formard.

Thanks Joe