diff --git a/src/pairs/sim/interaction.py b/src/pairs/sim/interaction.py
index 730ef04fba526f5b69d834e0b2127fff81ade549..f3aa66a362caf2b14ea1c0c3f37cf600cdbc4422 100644
--- a/src/pairs/sim/interaction.py
+++ b/src/pairs/sim/interaction.py
@@ -266,7 +266,7 @@ class ParticleInteraction(Lowerable):
                             Assign(self.sim, prop[i], prop[i] + reduction)
 
                         else:
-                            Assign(self.sim, prop[i], reduction)
+                            Assign(self.sim, prop[i], prop[i] + reduction)
 
         else:
             raise Exception("Interactions among more than two particles are currently not supported.")