[plt-scheme] debug for parser tools?

The help files for the parser-tools collection says

> (debug filename)
  ....
  Additionally, if a debug file is
     specified, when a running generated parser encounters a parse
     error on some input file, after the user specified error
     expression returns, the complete parse stack is printed to assist
     in debugging the grammar of that particular parser.
  ...

But I find that adding the (debug ...) option doesn't generate a stack
trace when an error occurs. I browsed through the code as well and it
doesn't seem like debug does anything more than dump the lalr table to
the file named. Am I missing something? I really need a debugging facility..