Catphan504 no HU module error

Hi. I have been playing around with Varian CBCT and pylinac (fantastic software written in my favourite programming language) and sometimes I get the following error:" No slices were found that resembled the HU linearity module." I am sure the module was there when a full phantom scanner was performed on my Clinac machine to my catphan504. I was not able to find what is causing this error but I have found the same problem in two threads of this forum. Nevertheless no answer, at least a public answer can be found in them. Do anybody know something more about this apparently low frequency error?
Thank you very much and congratulations for the pylinac software.

That error is a generic error that simply means that none of the slices had the characteristics resembling the HU slice, which is how pylinac localizes the phantom within the CT stack. This can happen for a number of reasons, but often happens when there are artifacts in the image. Do you have BBs on your phantom or do you scan the metal screws at the base of the phantom?