EXTERNAL database handles are not supported for RDB$INTERPRET

  Explanation:  External database handles are  not  supported  for
  RDB$INTERPRET.   Only  LOCAL and the default database handle are
  supported.    External   database   handles   apply   only    to
  preprocessors that can declare variables for database handles.

  User Action:  For database handles  that  must  be  accessed  in
  multiple   modules,  use  LOCAL.   Otherwise,  use  the  default
  database handle.