Mark unknown area as free space in gmapping

edit

My gmapping launch is as below

<node pkg="gmapping" type="slam_gmapping" respawn="true" name="slam_gmapping" output="screen">
    <param name="scan" value="/scan"/>
    <param name="odom_frame" value="odom"/>
    <param name="base_frame" value="base_link"/>
    <param name="delta" value="0.1" />
    <param name="xmin" value="-50.0" />
    <param name="xmax" value="50.0" />
    <param name="ymin" value="-50.0" />
    <param name="ymax" value="50.0" />
    <param name="maxRange" value="40.0"/>
    <param name="maxUrange" value="40.0"/>
</node>

For the space where there is no obstacle within the maxRange, gmapping should mark the space as free space but it marks it as unknown. How do I make it mark this area as free space?