I'm currently working on a project which uses Box2D, when the player uses a certain key it should anchor to the ground. For that I use a weld joint, but when I add the joint the game will sometimes crash, it has a 1/10 change to crash. The error I recieve: Showing the controller Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 11 at org.jbox2d.dynamics.Island.add(Island.java:577) at org.jbox2d.dynamics.World.solve(World.java:1073) at org.jbox2d.dynamics.World.step(World.java:598) The code for adding the joints: WeldJointDef def = new WeldJointDef(); def.initialize(body, anchoredObject.body, body.getWorldCenter()); weldJoint = (WeldJoint) world.createJoint(def); I still get the error if I synchronize it