diff --git a/HySoP/hysop/operator/monitors/compute_forces.py b/HySoP/hysop/operator/monitors/compute_forces.py
index 3268aacaf3137840f3a221f95bbbfb06882c6431..44694fdb1a7a25f8774453d6a121bbda23c6a8a8 100644
--- a/HySoP/hysop/operator/monitors/compute_forces.py
+++ b/HySoP/hysop/operator/monitors/compute_forces.py
@@ -134,8 +134,6 @@ class DragAndLift(Monitoring):
         dt = simulation.timeStep
         ite = simulation.currentIteration
 
-        self.force[...] = 0.0
-
         # Synchro of ghost points is required for fd schemes
         self._synchronize(self.vd.data + self.wd.data)
 
@@ -143,6 +141,7 @@ class DragAndLift(Monitoring):
         # -1/(N-1) . d/dt int(x ^ w)
         if self._voc.isEmpty[self.topo]:
             self._buffer[...] = 0.0
+            self.force[...] = 0.0
         else:
             self._buffer = self._integrateOnBox(self._buffer)
             self.force[...] = -1. / dt * self._coeff * (self._buffer