environment variable 'LDS_MODEL' is not set while launching turtlebot3_bringup

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Hello,

I'm trying to follow this guide : https://emanual.robotis.com/docs/en/p..., but when I try to roslaunch turtlebot3_bringup turtlebot3_robot.launch, this message shows up:

RLException: while processing /home/gdg/catkin_ws/src/turtlebot3/turtlebot3_bringup/launch/turtlebot3_lidar.launch:
Invalid <arg> tag: environment variable 'LDS_MODEL' is not set. 

Arg xml is <arg name="lds_model" default="$(env LDS_MODEL)" doc="LDS MODEL [LDS-01, LDS-02]"/>

How can I set this LDS_MODEL? Am I missing something?