Any progress figuring this out? I’m running into the same problem myself. Introducing a line m = m[0] after the tensor conversion m = pt.as_tensor(m) (and doing the same for c) doesn’t seem to fix the problem.
For what it’s worth, the older example available in the version history does seem to work properly, inasmuch as PyMC uses the NUTS sampler when the code is run. The code in the current example leads to the Slice sampler being invoked instead, presumably because the gradient function is not evaluating correctly.
(I’m entirely a newbie at this stuff, so my assessment of the situation may be way off. Feel free to disregard my ideas about what’s going wrong or how to fix it.)