diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index 96a71ae074dbfdc372e4cf09ba7d044d5b932f6d..e176f2c0af60676ac20b16aaa42e24a1c7ef11d0 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -207,7 +207,7 @@ class OhmPiHardware:
     @property
     def last_dev(self):
         if len(self.readings) > 1:
-            return 100. * np.std(self.readings[:, 2] * self.readings[:, 4] / self.readings[:, 3])/self.last_rho()
+            return 100. * np.std(self.readings[:, 2] * self.readings[:, 4] / self.readings[:, 3])/self.last_rho
         else:
             return np.nan