diff --git a/homeassistant/components/sensor/systemmonitor.py b/homeassistant/components/sensor/systemmonitor.py
index 0963786b7d0d1d68fc8f9959c35a4637e2634eb0..14eadb48984ebf8820e82f08728c19d0f1234ec8 100755
--- a/homeassistant/components/sensor/systemmonitor.py
+++ b/homeassistant/components/sensor/systemmonitor.py
@@ -10,7 +10,7 @@ import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 from homeassistant.const import (
-    CONF_RESOURCES, STATE_OFF, STATE_ON, CONF_TYPE)
+    CONF_RESOURCES, STATE_OFF, STATE_ON, STATE_UNKNOWN, CONF_TYPE)
 from homeassistant.helpers.entity import Entity
 import homeassistant.helpers.config_validation as cv
 import homeassistant.util.dt as dt_util
@@ -49,6 +49,18 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
         })])
 })
 
+IO_COUNTER = {
+    'network_out': 0,
+    'network_in': 1,
+    'packets_out': 2,
+    'packets_in': 3,
+}
+
+IF_ADDRS = {
+    'ipv4_address': 0,
+    'ipv6_address': 1,
+}
+
 
 # pylint: disable=unused-argument
 def setup_platform(hass, config, add_devices, discovery_info=None):
@@ -126,20 +138,25 @@ class SystemMonitorSensor(Entity):
                 self._state = STATE_ON
             else:
                 self._state = STATE_OFF
-        elif self.type == 'network_out':
-            self._state = round(psutil.net_io_counters(pernic=True)
-                                [self.argument][0] / 1024**2, 1)
-        elif self.type == 'network_in':
-            self._state = round(psutil.net_io_counters(pernic=True)
-                                [self.argument][1] / 1024**2, 1)
-        elif self.type == 'packets_out':
-            self._state = psutil.net_io_counters(pernic=True)[self.argument][2]
-        elif self.type == 'packets_in':
-            self._state = psutil.net_io_counters(pernic=True)[self.argument][3]
-        elif self.type == 'ipv4_address':
-            self._state = psutil.net_if_addrs()[self.argument][0][1]
-        elif self.type == 'ipv6_address':
-            self._state = psutil.net_if_addrs()[self.argument][1][1]
+        elif self.type == 'network_out' or self.type == 'network_in':
+            counters = psutil.net_io_counters(pernic=True)
+            if self.argument in counters:
+                counter = counters[self.argument][IO_COUNTER[self.type]]
+                self._state = round(counter / 1024**2, 1)
+            else:
+                self._state = STATE_UNKNOWN
+        elif self.type == 'packets_out' or self.type == 'packets_in':
+            counters = psutil.net_io_counters(pernic=True)
+            if self.argument in counters:
+                self._state = counters[self.argument][IO_COUNTER[self.type]]
+            else:
+                self._state = STATE_UNKNOWN
+        elif self.type == 'ipv4_address' or self.type == 'ipv6_address':
+            addresses = psutil.net_if_addrs()
+            if self.argument in addresses:
+                self._state = addresses[self.argument][IF_ADDRS[self.type]][1]
+            else:
+                self._state = STATE_UNKNOWN
         elif self.type == 'last_boot':
             self._state = dt_util.as_local(
                 dt_util.utc_from_timestamp(psutil.boot_time())