diff --git a/hardware_system.py b/hardware_system.py
index 0c60657ae43c34d70a21ad360d1adf59f62024f3..177733cd19ac8e9e800145e745af1ffa0fa5e706 100644
--- a/hardware_system.py
+++ b/hardware_system.py
@@ -118,7 +118,7 @@ class OhmPiHardware:
 
     @property
     def sp(self):
-        if len(self.readings[self.readings[:,2]==1, :]) < 1 or len(self.readings[self.readings[:,2]==-1, :]) < 1:
+        if self.readings.shape == (0,) or len(self.readings[self.readings[:,2]==1, :]) < 1 or len(self.readings[self.readings[:,2]==-1, :]) < 1:
             self.exec_logger.warning('Unable to compute sp: readings should at least contain one positive and one negative pulse')
             return 0.
         else: