diff --git a/wally/sensors/protocol.py b/wally/sensors/protocol.py
index 7688f31..a67ddde 100644
--- a/wally/sensors/protocol.py
+++ b/wally/sensors/protocol.py
@@ -104,8 +104,8 @@
             vals = [data[header].value - self.prev.get(header, 0)
                     for header in self.headers]
 
-            self.prev.update({header: data[header].value
-                              for header in self.headers})
+            self.prev.update(dict((header, data[header].value)
+                             for header in self.headers))
         else:
             vals = [data[header].value for header in self.headers]