Starting with version 1.50, a multi-configuration project with one axis of type Slaves, with the label "os" fails to build on Windows with the output:

Started by upstream project "TestMatrix" build number 4
originally caused by:
Started by user anonymous
[EnvInject] - Loading node environment variables.
Building on master in workspace C:\Programs\jenkins\jobs\TestMatrix\workspace\os\win
[win] $ mvn.bat -Dos=win install
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Invalid task 'win': you must specify a valid lifecycle phase, or a goal in the format plugin:goal or pluginGroupId:pluginArtifactId:pluginVersion:goal
[INFO] ------------------------------------------------------------------------
[INFO] For more information, run Maven with the -e switch
[INFO] ------------------------------------------------------------------------
[INFO] Total time: < 1 second
[INFO] Finished at: Fri Feb 01 13:15:25 PST 2013
[INFO] Final Memory: 1M/15M
[INFO] ------------------------------------------------------------------------
Build step 'Invoke top-level Maven targets' marked build as failure
Finished: FAILURE

mvn.bat almost seems to interpret "-Dos=win" as "-Dos= win" (two separate arguments). The same command works from the command-line.

To reproduce, create a new Multi-configuration project. Select Add Axis > Slaves. Set the label to "os" and select a node labeled "win". Select Add Build Step > Invoke Top level maven target. Save and build.