tcp::socket::is_open() is true after socket is closed

tcp::socket::is_open() is true after socket is closed

In the following simple code:
tcp::socket socket(ios);
socket.connect(...);
const bool iso1 = socket.is_open();
socket.close();
const bool iso2 = socket.is_open();
I get: iso1 == iso2 == true
Why? Thanks.
P.S. Ubuntu 13.04, boost-1.54.0