If I check a reiserfs file system for inodes with zabbix agent in percent (i.e. vfs.fs.inode[/,pfree]), I get the '-nan' in response.
Due to reiserfs does not has fixed predefined number of inodes on the file system, the resposne should be, I guess, something like 'ZBX_NOTSUPPORTED' or something more informative than just '-nan'.

Second, if I check for inodes like this 'vfs.fs.inode[/,free]', I get just zero in response.
Okay, man 2 statfs says 'Fields that are undefined for a particular file system are set to 0'. Why not to sent answer by zabbix agent like 'not supported' if the file system I try to check does not has fixed predefined number of inodes?