I'm writing an XNA game in C# using the XNA port of Box2d - Box2dx.

Entities like trees or zombies are represented as GameObjects. GameObjectManager adds and removes them from the game world:

    /// <summary>
    /// Does the work of removing the GameObject.
    /// </summary>
    /// <param name="controller">The GameObject to be removed.</param>
    private void removeGameObjectFromWorld(GameObjectController controller)
    {
        controllers.Remove(controller);
        worldState.Models.Remove(controller.Model);
        controller.Model.Body.SetActive(false);
    }

    public void addGameObjectToWorld(GameObjectController controller)
    {
        controllers.Add(controller);
        worldState.Models.Add(controller.Model);
        controller.Model.Body.SetActive(true);
    }

controllers is a collection of GameObjectController instances.

worldState.Models is a collection of GameObjectModel instances.

When I remove GameObjects from Box2d this way, this method gets called:

       void IContactListener.EndContact(Contact contact)
        {
            GameObjectController collider1 = worldQueryUtils.gameObjectOfBody(contact.GetFixtureA().GetBody());
            GameObjectController collider2 = worldQueryUtils.gameObjectOfBody(contact.GetFixtureB().GetBody());
            collisionRecorder.removeCollision(collider1, collider2);
}

worldQueryUtils:

    // this could be cached if we know bodies never change
    public GameObjectController gameObjectOfBody(Body body)
    {
        return worldQueryEngine.GameObjectsForPredicate(x => x.Model.Body == body).Single();
    }

This method throws an error:

System.InvalidOperationException was unhandled
  Message="Sequence contains no elements"
  Source="System.Core"
  StackTrace:
       at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
       at etc

Why is this happening? What can I do to avoid it? This method has been called many times before the body.SetActive() was called. I feel that this may be messing it up.