python exception "nfsiostat:217:compare_iostats:KeyError: 'NULL'"

Issue

• python exception "nfsiostat:217:compare_iostats:KeyError: 'NULL'" is met on nfsiostat

$ cat backtrace 
nfsiostat:217:compare_iostats:KeyError: 'NULL'

Traceback (most recent call last):
  File "/usr/sbin/nfsiostat", line 649, in <module>
    iostat_command(prog)
  File "/usr/sbin/nfsiostat", line 617, in iostat_command
    print_iostat_summary(old_mountstats, mountstats, devices, sample_time, options)
  File "/usr/sbin/nfsiostat", line 468, in print_iostat_summary
    diff_stats[device] = stats[device].compare_iostats(old_stats)
  File "/usr/sbin/nfsiostat", line 217, in compare_iostats
    difference, self.__rpc_data[op], old_stats.__rpc_data[op]))
KeyError: 'NULL'

Local variables in innermost frame:
value: [0, 0, 0, 0, 0, 0, 0, 0]
self: <__main__.DeviceData instance at 0x7f0b0464e440>
old_stats: <__main__.DeviceData instance at 0x7f0b04659a70>
result: <__main__.DeviceData instance at 0x7f0b04659a28>
key: 'RMDIR'
op: 'NULL'

• The above exception is met when running /usr/bin/python /usr/sbin/nfsiostat 60 2 -p