What value is given to constant in Scipy splantider?

Question

Im using BSplines from SciPy and computing their antiderivatives using the antiderivative() method of the BSpline object. The antiderivative() method computes the antiderivative through a call to the funciton splantider(). My code works fine and does what it needs to do using the splines and their antiderivatives. However, I keep wondering what value is given to the constant value C, i.e. the standard + C in indefinite integration, that arises when computing the antiderivative using these methods.

Given the results of experiments I've done with my code I would assume the value is 0, I've tried looking around the SciPy source code of these methods as well as the work of De Boor (2001) but didn't find anything. I'm also unsure if I can check this programmatically, so any tips there are helpful.

So my question is:

What value is given to the added constant in the antiderivative in SciPy when computing a BSpline antiderivative using the antiderivative() method?