nan value after changing inertia matrix [closed]

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Hello everybody,

I am the following problem with my robot (rear wheels don't touch the ground):

So I have changed the inertia of my links but I got an error when I change the matrix of some wheels (it's strange because the link in the error message is not linked to the wheels involved in the problem):

[ERROR] [1453136122.044577471, 24.646000000]: Ignoring transform for child_frame_id "fourche" from authority "unknown_publisher" because of a nan value in the transform (-nan -nan -nan) (0.000000 0.000000 0.000000 1.000000)

Urdf description of my wheels (diameter: 125mm, height: 50mm) :

<link name="${prefix}_motor_wheel">
        <gazebo>
            <self_collide>false</self_collide>
        </gazebo>
        <visual>
            <material name="Yellow">
                <color rgba="0.8 0.8 0.0 1.0"/>
            </material>
            <geometry>
                <mesh filename="package://agv/urdf/model/motor_wheel_left.dae"/>
            </geometry>
        </visual>
        <collision>
            <geometry>
                <mesh filename="package://agv/urdf/model/motor_wheel_left.dae"/>
            </geometry>
        </collision>
        <inertial>
            <mass value="1.8"/>
            <inertia
            ixx="0.0021" ixy="0.0" ixz="0.0"
            iyy="0.0035" iyz="0.0"
            izz="0.0021"/>
        </inertial>
        <!--<inertial>
            <mass value="1"/>
            <inertia
            ixx="1.0" ixy="0.0" ixz="0.0"
            iyy="1.0" iyz="0.0"
            izz="1.0"/>
        </inertial>-->
    </link>

Thanks in advance