Occasionally, the workspace cleanup plugin stops a build on a null value.

The console output is as follows:
2013-05-29 16:29:54.920 Started by user subversion
2013-05-29 16:29:54.994 [EnvInject] - Loading node environment variables.
2013-05-29 16:29:54.995 Building in workspace D:\Jenkins\jobs\WxSimSft\workspace
2013-05-29 16:29:54.995
2013-05-29 16:29:54.995 Deleting project workspace...
2013-05-29 16:30:11.319 Cannot delete workspace: null
2013-05-29 16:30:11.322 ERROR: Cannot delete workspace: null
2013-05-29 16:30:11.324 Finished: FAILURE

This output was with Jenkins 1.515, but I've noticed it on previous versions as well. Building the project again usually clears the issue.