nan in laser scan data

edit

I'm trying to run gmapping on my turtlebot 2 but I always get the error "scan matching failed, using odometry" and the results are awful. I echoed the /scan topic and half of the laserscan data was "nan". My thought is that my scan increment does not increment fully through my scan range such that the scan only picks up half of the laserscan data within the scan range.

Can anyone verify this and tell me how to change the scan range/scan increment parameters?

Thanks!!