teamcity.build.checkoutDir is empty

I'm using the sln2008 buildrunner and try to use the teamcity.build.checkoutDir TeamCity property.

To test it I logged a message in my build log:

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In the build log the following appeared:
teamcity.build.checkoutDir =

I expected to see something like:
C:\Program Files\TeamCity\buildAgent\work\e3f031c64e73354c

Can someone give a clue? Can I use the property as stated above?

Kind regards,

Michel Schep