diff --git a/ohmpi/hardware_system.py b/ohmpi/hardware_system.py
index c3c73c4899f936ae24e78dc55bdfd36dc5dd5f54..f46c517b0e24735bf8114e71197c6851a2c93b76 100644
--- a/ohmpi/hardware_system.py
+++ b/ohmpi/hardware_system.py
@@ -404,7 +404,7 @@ class OhmPiHardware:
         assert 0. <= duty_cycle <= 1.
         if duty_cycle < 1.:
             durations = [cycle_duration/2 * duty_cycle, cycle_duration/2*(1.-duty_cycle)] * 2 * cycles
-            pol = [-self.tx.polarity * np.heaviside(i % 2, -1.) for i in range(2 * cycles)]
+            pol = [-int(self.tx.polarity * np.heaviside(i % 2, -1.)) for i in range(2 * cycles)]
             polarities = [0] * (len(pol) * 2)
             polarities[0::2] = pol
         else: