TFS 2010 Branching Error on Label TF10169

I can't create a branch in TFS; when I try to 开发者_JS百科branch the code in $/MainCode to $/BranchCode, using a label as the source, I get this error:

TF10169: Unsupported pending change attempted on team project folder $/BranchCode. Use the Project Creation Wizard in Team Explorer to create a project or the Team Project deletion tool to delete one.

Why would I need to create or delete a project? Both $/MainCode and $/BranchCode exist....... I don't get this.

Thanks.

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I might be completely wrong here, but isn't it the case that the things immediately below $/ are Team Projects, rather than branches? So you would have

$
    SuperFoo
        Main
        vNext
    AmazingBar
        Main
        vNext
        vNextNext

and you would branch (say) $/SuperFoo/Main to $/SuperFoo/MyNewBranch ?

I can't right now find decent docs on Team Projects, and my experience is only with TFS 2005, but that's how I've always understood the hierarchy.