diff --git a/pyrobolearn/worlds/world.py b/pyrobolearn/worlds/world.py
index 5b9c4f0..0ab556f 100644
--- a/pyrobolearn/worlds/world.py
+++ b/pyrobolearn/worlds/world.py
@@ -391,7 +391,8 @@ class World(object):
 
         # call the step method for each body
         for body in self.bodies.values():
-            body.step()
+            if isinstance(body, Body):
+                body.step()
 
         # call simulation step
         self.sim.step()