Cell widget does not span correctly when increasing the column span

#include <QtGui>

class TableWidget : public QTableWidget
{
	Q_OBJECT
public:
	TableWidget()
	{
		button1 = new QPushButton("Click me", this);
		setRowCount(1);
		setColumnCount(1);
		setCellWidget(0, 0, button1);
		connect(button1, SIGNAL(clicked()), this, SLOT(test1()));
	}
	public slots:
		void test1()
		{
			insertColumn(columnCount());
			setSpan(0,0,1,columnCount());

		}
private:
	QPushButton *button1;
};

#include "main.moc"

int main(int argc, char** argv)
{
	QApplication app(argc, argv);
	TableWidget window;
	window.show();
	return app.exec();

}