p273节，在使用rosrun gazebo_ros spawn_model -urdf -model car -param robot_description命令的时候报错了

采用地操作系统:ubuntu18.04

问题

p273节，在使用rosrun gazebo_ros spawn_model -urdf -model car -param robot_description命令的时候报错了
Error: Inertial element must have inertia element
at line 313 in /build/urdfdom-UJ3kd6/urdfdom-0.4.1/urdf_parser/src/link.cpp
Error: Could not parse inertial element for Link 【base_link】
at line 434 in /build/urdfdom-UJ3kd6/urdfdom-0.4.1/urdf_parser/src/link.cpp

urdf文件：

<robot name="mycar">
<link name="base_link">
<visual>
<geometry>
<box size="0.5 0.2 0.1" />
。。。
<inertial>
<origin xyz="0 0 0" />
<mass value="6" />
<inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1" />
</inertial>
</link>
<gazebo reference="base_link">
<material>Gazebo/Black</material>
</gazebo>

</robot>