I happened to have `shopt -s extdebug` in my .bashrc recently. After a new bash session started, I got the following output.
bash debugger, bashdb, release 4.2-0.8 Copyright 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010, 2011 Rocky Bernstein This is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. ** Internal debug error _Dbg_is_file(): file argument null bash: _Dbg_filenames[$fullname]: bad array subscript I guess `--debugger` or `shopt -s extdebug` is not supposed to be run under interactive mode? If so, maybe a detection should be done for this situation?