From https://github.com/lucee/Lucee/pull/499 (tho PR is against 5.2.7)
Code references missing "existing" variable which should pre-load mapping sittings.
manually adapted the change (a little bit different)
please give this a try and create a test case if not already exists
I've checked this ticket and works fine with the fixed version. And also added a test case for this ticket.
Pull Request: https://github.com/lucee/Lucee/pull/1040