Based on Michele's testing, it sounds like the core of the issue might be that whatever function attempts to normalize the callnumber sortkey is unable to handle a null value on the callnumber label - so likely an addition to the existing functionality, rather than an error in the code as it currently stands. My guess is that the normalize functionality expects a bare minimum of having the callnumber label in order to work
Based on Michele's testing, it sounds like the core of the issue might be that whatever function attempts to normalize the callnumber sortkey is unable to handle a null value on the callnumber label - so likely an addition to the existing functionality, rather than an error in the code as it currently stands. My guess is that the normalize functionality expects a bare minimum of having the callnumber label in order to work