arbtt 0.6 and xmonad: no windows list?

[Timothee BESSET]

Hello,

If I use xmonad as my window manager (http://xmonad.org/), arbtt is
not collecting any active windows information:

..
TimeLogEntry {tlTime = 2011-02-20 03:19:50.706574 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 261}}
TimeLogEntry {tlTime = 2011-02-20 03:20:50.788981 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 11928}}
TimeLogEntry {tlTime = 2011-02-20 03:21:50.869605 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 26}}
TimeLogEntry {tlTime = 2011-02-20 03:22:50.949739 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 19}}
TimeLogEntry {tlTime = 2011-02-20 03:23:51.030388 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 29}}
TimeLogEntry {tlTime = 2011-02-20 03:24:51.109929 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 0}}
TimeLogEntry {tlTime = 2011-02-20 03:25:51.190305 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 3966}}
TimeLogEntry {tlTime = 2011-02-20 03:26:51.271169 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 816}}
TimeLogEntry {tlTime = 2011-02-20 03:27:51.373603 UTC, tlRate = 60000,
tlData = CaptureData {cWindows = [], cLastActivity = 121}}
..

When I start arbtt-capture, I also receive the following message:

arbtt: ERROR: No _NET_CLIENT_LIST set for the root window

Is that the cause of the problem, and how do I fix it?

Best,
TTimo