diff --git a/platypush/entities/sensors.py b/platypush/entities/sensors.py
index 84da7aff..0a2c0986 100644
--- a/platypush/entities/sensors.py
+++ b/platypush/entities/sensors.py
@@ -53,6 +53,7 @@ if 'raw_sensor' not in Base.metadata:
         If ``is_json`` is ``True``, then ``value`` is a JSON-encoded string
         object or array.
         """
+        unit = Column(String)
 
         @property
         def value(self):