This means that the system can't find any x-values to substitute where the expression would be defined.
In your example, the domain of $ans is [0,1], so all of your test points are outside of this domain.
There are a couple ways to fix this:
- Replace your test points with values between 0 and 1
- Replace "test_points=>..." with "limits=>[0,1]", which would force WeBWorK to choose x-values between 0 and 1 for evaluation.
There is more info about domains and such at http://webwork.maa.org/wiki/FormulaDomain1