Problem operating LAStools in QGIS 2.6.1

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 (http://ift.tt/1nB72bC). 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. I downloaded a LiDAR tile from the National Land Survey of Finland (http://ift.tt/12scntR) 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? Thanks,Richard