Infinite loop with pgf.ObjectValid

Why would this result in an infinite loop? I thought that wehn it got to the last Pgf object it should become invalid and stop.

    var doc = app.ActiveDoc;

    var pgf = doc.MainFlowInDoc.FirstTextFrameInFlow.FirstPgf;

    while ( pgf.ObjectValid() ) {

        Console('Hello');

    }

    pgf = pgf.NextPgfInFlow;

Thanks,

Mark