Why does my QWidget stylesheet not recognise objectName rules?

Here I have a subclass of QComboBox which is going to change the colour of its text according to the validity of its contents.

namespace{    const QString GOOD = "good";    const QString BAD  = "bad";}NewDBLabel :: NewDBLabel( DB db, const QString & table ) :    m_db (db){    connect (this, SIGNAL (textChanged (const QString &)),            this,  SLOT   (changed     (const QString &)));    setStyleSheet ("QWidget#" + GOOD +" {color: black;} ""QWidget#" + BAD  +" {color: darkRed;}");}void NewDBLabel :: changed (const QString & label){    if (m_db .label_exists (m_table, label))    {        qWarning ("exists"); // this is printed correctly        setObjectName (BAD);        emit valid (false);    }    else    {        qWarning ("new"); // this is printed correctly        setObjectName (GOOD);        emit valid (true);    }}

As far as I can tell, this conforms to the documentation: QWidget#foo should match subclasses of QWidget whose objectName is foo.

I expected that setObjectName would cause the different style rules to be applied, but this doesn't happen. Any idea why?