As a Sicilian, I can’t forego the opportunity of using a package with such name! 
This being said, please keep in mind that I’m a total beginner in bundle adjustment, nonlinear optimization, and possibly more of the maths involved. I’ll do it for sure as soon as I get the chance, just don’t expect a paper from me any time soon 
Ah! Well, I did have that problem (with a more explicit error message), but I think I solved it… I called register for project, distance_sq and their dependencies. It’s possible to see that in the gist. After that, it switched to complaining about the array in a nonlinear expression. Thank you, nevertheless; I’ll check the code again as soon as I get to my laptop again.