Access to a property for a non-object value

_book_access_get_book_nid() has the following code:

function _book_access_get_book_nid($nid) {
  $result = db_query("SELECT bid FROM {book} WHERE nid = %d", $nid);
  return db_fetch_object($result)->bid;
}

As db_fetch_object() can return FALSE, the return line tries to access a property for a value that is not an object.