diff --git a/homeassistant/helpers/entity.py b/homeassistant/helpers/entity.py
index 0f5ccf78bc78b7225354b22678d14628177d5cdc..742b877946a84e7e2fe731506d1e5f88b7f91c55 100644
--- a/homeassistant/helpers/entity.py
+++ b/homeassistant/helpers/entity.py
@@ -145,7 +145,7 @@ class Entity(object):
         if force_refresh:
             self.update()
 
-        state = str(self.state)
+        state = STATE_UNKNOWN if self.state is None else str(self.state)
         attr = self.state_attributes or {}
 
         device_attr = self.device_state_attributes