error - Using LAStools in QGIS gives "global name 'os' is not defined"?

I have a problem with LAStools in QGIS 2.6.1. I have never used QGIS to process LiDAR data before.

I downloaded LAStools yesterday from the rapidlasso GmbH website. I unzipped the “lastools.zip” file into my osgeo folder (C:\OSGeo4W64\LAStools) and followed the instructions for activating LAStools and specifying the folder location. LAStools appears correctly in my Processing Toolbox in QGIS.

Then, I downloaded a LiDAR tile from the National Land Survey of Finland as a .laz file. I tried to use lasground to view this data, and followed the instructions on the rapidlasso GmbH website (see screengrab). However, I get this message in my log:

Algorithm lasground starting...global name 'os' is not defined See log for more details

Am I missing some dependency from my library?