Why do I get no line numbers from a stack trace created from Exceptions?

Okay; assuming this code running in debug mode -

static StackFrame GetTopFrameWithLineNumber(Exception e)
{
    StackTrace trace = new StackTrace(e);
    foreach (StackFrame frame in trace.GetFrames())
    {
        if (frame.GetFileLineNumber() != 0)
        {
            return frame;
        }
    }
    return null;
}

I'm ALWAYS returning null. Why do the stack frames have no line numbers when if I inspect the Exception.StackTrace string, it clearly does have them for any non-framework code? Is there some issue with constructing a stack trace from an exception that i'm not aware of?

EDIT FOR CLARITY: In the thrown exception I can see the line numbers in the StackTrace property. I'm assuming that means I have everything else I need.