Can not start ntop. After start ntop I have the following problem:

Mon Mar 18 22:05:59 2013 NOTE: Interface merge enabled by default
Mon Mar 18 22:05:59 2013 Initializing gdbm databases
Mon Mar 18 22:05:59 2013 **ERROR** ....open of /var/ntop/prefsCache.db failed: File open error
Mon Mar 18 22:05:59 2013 Possible solution: please use '-P <directory>'
Mon Mar 18 22:05:59 2013 **FATAL_ERROR** GDBM open failed, ntop shutting down...
Mon Mar 18 22:05:59 2013 CLEANUP[t140707982616768]: ntop caught signal 2 [state=2]
Mon Mar 18 22:05:59 2013 ntop is now quitting...