user defined value is not set in ~50% of cases of linux users.
https://www.google.com/analytics/web/?hl=en#report/content-pages/a17645367w35680699p35061163/%3F_u.date00%3D20120405%26_u.date01%3D20150505%26_.sampleSize%3D500000%26_.useg%3Duser4MedbQIpSGmZCXZSz373WQ%26explorer-segmentExplorer.segmentId%3Danalytics.userDefinedValue%26explorer-table.plotKeys%3D[]/