problem with treeview

My app calls TreeView_InsertItem, which leads to a crash in ntdll.dll.
A backtrace is available at http://www.lustbader.net/bt.txt, and I can
provide any other debug output if anyone thinks it will help.
TreeView_InsertItem is called like this:

TVITEM tviFoo;
tviFoo.mask = TVIF_TEXT | TVIF_PARAM;
tviFoo.pszText = LPSTR_TEXTCALLBACK;
tviFoo.lParam = (LPARAM)pFoo;  //pFoo is guaranteed to be non-NULL
TVINSERTSTRUCT tvinsFoo;
tvinsFoo.hParent = TVI_ROOT;
tvinsFoo.hInsertAfter = TVI_LAST;
tvinsFoo.item = tviFoo;
HTREEITEM hTreeFoo = TreeView_InsertItem(m_hWndUserTree, &tvinsFoo);

Any ideas?

-Steve