Error 500 (internal server error) in the comparison page

the comparison page between https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/1737/ and https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master-mr/5019/ fails with error 500 (https://lhcb-nightlies.web.cern.ch//compare?a=nightly/lhcb-master/1737&b=nightly/lhcb-master-mr/5019).

The logs report:

ERROR:lb.nightly.frontend.application:Exception on /compare [GET]
Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 2073, in wsgi_app
response = self.full_dispatch_request()
File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1518, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1516, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1502, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/opt/web-app/lb/nightly/frontend/compare.py", line 116, in compare
{
File "/opt/web-app/lb/nightly/frontend/compare.py", line 117, in <dictcomp>
k: set(v)
TypeError: 'NoneType' object is not iterable