TFS SourcesDirectory is null/empty when used in Expression Editor

I have a build definition which has two working folders specified. One of these has the following columns:

• Status: Active
• Source Control Folder: $/MyCompany/MyDivision/Solution2
• Build Agent Folder: $(SourceDir)\Solution2

I am trying to access this Build Agent Folder from within the Expression editor in TFS 2012?

I've tried SourcesDirectory but when it gets evaluated it is null/empty. Is there something specific I need to do to get SourcesDiretory to work in the expression editor? Or is there a better way to reference this specific build agent folder in it's entirety?

The SourcesDirectory and indeed the BinariesDirectory are not set till later in the build. They are not immediately available at the start of the build.