When invoking TFPT you might get one of the errors:

 

Unable to determine the workspace

Unable to determine the source control server

 

To get around this try invoking tfpt from within a directory that is already mapped to Team Foundation source control. 

If that doens't do it your local workspace cache may need to be updated. This can done via, tf workspaces /s:<server URL>

 

Thanks to on of the TFS Developers, Ben Ryan, for this pointer!