VSTO With WPF: Index not found (0x80131124) error when adding addition, empty method

I'm working on a VSTO Outlook plug in that has WPF windows. When writing the view model, simply adding an additional method results in an "Index not found. (Exception from HRESULT: 0x80131124)" compiler error. If I remove the empty method, the error goes away. Here is the class without the compiler error:

public class ImportViewModel : ViewModelBase
{
    public event EventHandler CloseRequested;

    private ICommand _closeRequestedCommand;
    private ICommand _selectOutlookFolderCommand;
}

And here's my class with the compiler error:

public class ImportViewModel : ViewModelBase
{
    public event EventHandler CloseRequested;

    private ICommand _closeRequestedCommand;
    private ICommand _selectOutlookFolderCommand;

    private void SelectOutlookFolder()
    {
    }
}

Nothing else has changed between the two.