Z-coordinate has to be 1 or -1

Hello, I'm starting to build a map using gmapping, but whenever I start slam_gmapping, the following warning is shown: Laser has to be mounted planar! Z-coordinate has to be 1 or -1, but gave: 0.00000 This is one of the published transformations: When I change the X argument into 1 or -1 the warning is gone! I don't know why!.But I'm sure there is something wrong!! and I need the X to be 0. any idea? UPDATE: These are the frames in rviz:  and I cannot change the values of the orientation in rviz.