SimpleXML Error: Node no longer exists (using addChild())

[b.coolsaet]

Hi,

like the title says simplexml is giving me a "Node no longer exists"
error while using the addChild() function. Note that it's not giving
an error on the first addChild();

Can somebody help ?
code causing error:

$xml = simplexml_load_ file("../xml/books.xml");

if($xml) {

$book = $xml->books->addChild('book ');
$book->addChild('titl e',$this->myTitle); //
line 171
$book->addChild('subt itle',$this->mySubTitle);
$book->addChild('publ isher',$this->myPublisher) ;
$book->addChild('year ',$this->myYear);
$book->addChild('cove r',$this->cover);
$book->addChild('cove r_thumb',$this->cover_thumb) ; // line 176
}

xml structure:

<books>
<book>
<title></title>
<subtitle></subtitle>
<publisher></publisher>
<year></year>
<cover></cover>
<cover_thumb> </cover_thumb>
</book>
</books>

complete error:

Warning: SimpleXMLElemen t::addChild() [function.Simple XMLElement-
addChild]: Node no longer exists in C:\Program Files\XAMPP\xam pp\htdocs
\php\Book.class .php on line 171

Warning: SimpleXMLElemen t::addChild() [function.Simple XMLElement-
addChild]: Node no longer exists in C:\Program Files\XAMPP\xam pp\htdocs
\php\Book.class .php on line 172

Warning: SimpleXMLElemen t::addChild() [function.Simple XMLElement-
addChild]: Node no longer exists in C:\Program Files\XAMPP\xam pp\htdocs
\php\Book.class .php on line 173

Warning: SimpleXMLElemen t::addChild() [function.Simple XMLElement-
addChild]: Node no longer exists in C:\Program Files\XAMPP\xam pp\htdocs
\php\Book.class .php on line 174

Warning: SimpleXMLElemen t::addChild() [function.Simple XMLElement-
addChild]: Node no longer exists in C:\Program Files\XAMPP\xam pp\htdocs
\php\Book.class .php on line 175

Warning: SimpleXMLElemen t::addChild() [function.Simple XMLElement-
addChild]: Node no longer exists in C:\Program Files\XAMPP\xam pp\htdocs
\php\Book.class .php on line 176