diff --git a/advtrains/wagons.lua b/advtrains/wagons.lua
index b30d3d4..3e73f39 100644
--- a/advtrains/wagons.lua
+++ b/advtrains/wagons.lua
@@ -519,7 +519,7 @@ function wagon:on_step(dtime)
 				self:update_animation(gp.velocity, self.old_velocity)
 			end
 			if self.custom_on_velocity_change then
-				self:custom_on_velocity_change(gp.velocity, self.old_velocity)
+				self:custom_on_velocity_change(gp.velocity, self.old_velocity or 0)
 			end
 		end