KeyError in ipdevpoll psu plugin

ipdevpoll inventory job is consistently crashing on a couple of Cisco routers on a client installation, with the following traceback:

[ERROR jobs.jobhandler] [inventory some-gw.example.org] Plugin nav.ipdevpoll.plugins.psu.PowerSupplyUnit(u'some-gw.example.org') reported an unhandled failure
Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 345, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 424, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 441, in _runCallbacks
    self.result = callback(self.result, *args, **kw)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 949, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 891, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.6/dist-packages/twisted/python/failure.py", line 338, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/pymodules/python2.6/nav/ipdevpoll/plugins/psu.py", line 140, in handle
    entity_index)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 891, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.6/dist-packages/twisted/python/failure.py", line 338, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/pymodules/python2.6/nav/mibs/cisco_entity_fru_control_mib.py", line 118, in is_psu_up
    self.psu_status_table = yield self._get_power_status_table()
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 891, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.6/dist-packages/twisted/python/failure.py", line 338, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/pymodules/python2.6/nav/mibs/cisco_entity_fru_control_mib.py", line 62, in _get_power_status_table
    table = yield self._get_named_table('cefcFRUPowerStatusTable')
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 891, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.6/dist-packages/twisted/python/failure.py", line 338, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/pymodules/python2.6/nav/mibs/cisco_entity_fru_control_mib.py", line 49, in _get_named_table
    named_table = yield df
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 441, in _runCallbacks
    self.result = callback(self.result, *args, **kw)
  File "/usr/lib/pymodules/python2.6/nav/mibs/mibretriever.py", line 436, in resultFormatter
    column_name = table.reverse_column_index[column_no]
exceptions.KeyError: 4