Fragment#getAssociatedMarkupStream, under certain conditions, decides to ask its parent for the AssociatedMarkupStream.
To do so, it uses the #getParent() method.
This is not always correct, because the Fragment's parent is not always a container with associated markup (for example, it could be a Form).
The problem can be easily fixed by using the #findParentWithAssociatedMarkup() method instead.
(See org.apache.wicket.markup.html.panel.Fragment.java, lines 344 and 346 in wicket 1.3.5)