I follow the tutorial [turtlebot_simulator](http://wiki.ros.org/turtlebot_simulator/Tutorials/hydro/Make%20a%20map%20and%20navigate%20with%20it) to make a map in a simulated world.But when I run > roslaunch turtlebot_gazebo gmapping_demo.launch
a error that scan message must contain angles from -x to x occurs, as show below. How to deal with it.
`[ERROR] [1433813576.466253926, 24.270000000]: Scan message must contain angles from -x to x, i.e. angle_min = -angle_max
`
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