SetBinding(SearchBar.TextProperty ,.. ) does not work ?

SearchBar s = new SearchBar() { Placeholder = "From Port Name", ClassId = "fromport" };
s.SetBinding(SearchBar.TextProperty, "FromPort");

 Entry e = new Entry { };
 e.SetBinding(Entry.TextProperty, "FromPort");

In this little example, "e" gets updated with the "FromPort" value from the binding, but "s" does not? Why should binding on the Text element of an searchbar not work ?

Tested on Windows Phone 8 and Android.