Why does RVIZ complain that I have to transform from right_wheel to base_link?

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Hello,

Here is my output from check_urdf:

---------- Successfully Parsed XML ---------------
root Link: base_link has 1 child(ren)
    child(1):  chassis
        child(1):  left_wheel_assembly
            child(1):  left_wheel
        child(2):  right_wheel_assembly
            child(1):  right_wheel

When I open my model in RVIZ, I add a robot model, then add the TF, then specify my fixed frame (base_link). But I have an error under RobotModel saying "No transform from [left_wheel] to [base_link]" and "No transform from [right_wheel] to [base_link].

I'm not sure what's causing the issue and I'm running out of ideas. Can anyone give me advice? Thanks. Here is my launch file and urdf model.

bot_empty.launch

<launch> 
  <!-- these are the arguments you can pass this launch file, for example paused:=true --> 
  <arg name="paused" default="false"/> 
  <arg name="use_sim_time" default="true"/> 
  <arg name="gui" default="true"/> 
  <arg name="headless" default="false"/> 
  <arg name="debug" default="false"/> 
  <arg name="use_gui" value="true" />


    <!-- Load joint controller configuration from YAML file to parameter server -->
    <rosparam file="$(find ack_control)/config/ack_control.yaml" command="load" />

    <!-- load the controllers --> 
    <node name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false"
    output="screen" ns="/ack" args="joint_rwh_position_controller joint_lwh_position_controller joint_state_controller"/>



  <!-- convert joint states to TF transforms for rviz, etc -->
  <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"
    respawn="false" output="screen">
    <remap from="/joint_states" to="/ack/joint_states" />
  </node>

  <!-- Start Joint state publisher node which will publish the joint values -->
  <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" />


  <!-- Load the URDF into the ROS Parameter Server  
  <param name="robot_description" 
     command="$(find xacro)/xacro.py '$(find ack_description)/urdf/ackermann_1.urdf'" /> -->

  <param name="robot_description" textfile="$(find ack_description)/urdf/ackermann_1.urdf" />

  <include file="$(find gazebo_ros)/launch/empty_world.launch"> 
    <arg name="world_name" value="$(find ack_gazebo)/worlds/empty.world"/> 
    <arg name="debug" value="$(arg debug)" /> 
    <arg name="gui" value="$(arg gui)" /> 
    <arg name="paused" value="$(arg paused)"/> 
    <arg name="use_sim_time" value="$(arg use_sim_time)"/> 
    <arg name="headless" value="$(arg headless)"/> 
  </include>  

  <!-- Run a python script to the send a service call to gazebo_ros to spawn a URDF robot --> 

  <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model ackermann_1 -param robot_description"/>  

  <!-- Launch rviz -->
  <node name="rviz" pkg="rviz" type="rviz" 
    args="-d $(find ack_description)/urdf.rviz" required="true" />

</launch>

ackermann_1.urdf

<?xml version="1.0"?> 
<robot name="ackermann_1" xmlns:xacro="http://www.ros.org/wiki/xacro"> 

    <!-- Base link -->  
  <link name="base_link">
    <visual>  
      <geometry>
        <box size="0.01 0.01 0.01"/>
      </geometry>
    </visual>
  </link> 

    <!-- Chassis START-->  
    <joint name="base_joint" type="fixed">
    <parent link="base_link"/> 
    <child link="chassis"/> 
  </joint> 

  <link name="chassis"> 
    <collision> 
      <origin xyz="0 0 0.3" rpy="0 0 0"/> 
      <geometry> 
            <box size="1 0.2 0.1"/> 
      </geometry> 
    </collision> 
    <visual> ...