Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I am running gdb over lilypond, which is a very large C++ program with
embedded guile.  At a certain point, if I type `n', I end up with a dead
debugging environment, and a signalled error (wrong-type-argument
stringp nil) which comes from gud-find-file (on nil) via
gud-display-frame.

I am including the Backtrace after toggling debug-on-error, and the
value of gdb-debug-log.  If there is anything more I can do to debug
this problem, please let me know.