Problem with Rviz

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Hi, I want to see the simulation of what my Lego NXT is doing with rviz.
Therefore I created the model of my robot with Lego digital designer and the converted it in the right format, that is to say in a .urdf file.
I renamed my sensors and motors with the name that I specified in the .yaml file. But when I run rviz I get:

Robot Model
    Status Error
    ref_0_link No trasform from [ref_0_link] to [/world]
    ...

In my nodes I specified a tf between the robot base frame and the world frame, and another between the ultrasonic sensor frame and the robot frame.
In fact the links robot and ultrasonic_sensor are the only ones with Transform Ok.
I thought that because of the fact the all the links are children of robot and the joint are fixed there is no need to specify a tf for every link.
Can anyone tell me how to solve my problem?