Covariance always 0

[FrederikeDurow]

Hey,
I am trying to fuse my laser_scan_matcher and wheel odometry data with robot_localization.
For that I do need the covariance of the laser scan matcher, however it keeps on being 0 even though I set the do_compute_covariance parameter to 1.
I cannot seem to find any other parameters related to the covariance, but maybe I am missing something?
I would very much appreciate if any of you might have an idea of how to solve this issue.
I attatched my laser_scan_matcher.cpp script below.
Thank you in advance!

laser_scan_matcher.zip