Once a Polynomial kernel has been created with the default degree of zero, this can never be changed. Trivial example:

public static void Main(string[] args)
{
    var polynomial = new Polynomial();
    polynomial.Degree = 3;
}

Expected result: polynomial degree is changed to 3 as that is a valid value.
Actual result: System.ArgumentOutOfRangeException 'Degree must be positive'

Fairly obviously being caused by the check at Polynomial.cs:72:

if (degree <= 0)
    throw new ArgumentOutOfRangeException("value", "Degree must be positive.");

and the obvious fix would be to make the check be on value (the new value) rather than degree (the old value). I'll make a PR if you want one, but doesn't really seem worth it for such a small change.