Canvas and tooltips

[Juan Toledo <toledo users sourceforge net>]

Is it possible to set tooltips to canvas items?
Up until now I've been doing my own tooltips (which don't look really
nice), when I realized that you could supposedly use GtkTooltips on any
widget. But when I tried

 409    gtk_tooltips_set_tip (canvas_node->tooltips,
 410                          GTK_WIDGET(canvas_node->node_item),
 411                          str->str,
 412                          NULL);

I got

Gtk-CRITICAL **: file gtkwidget.c: line 4864 (gtk_widget_ref): assertion

`GTK_IS_WIDGET (widget)' failed.

Gtk-WARNING **: gtk_signal_connect(): could not find signal "unmap" in
the `GnomeCanvasEllipse' class ancestry

Gtk-WARNING **: gtk_signal_connect(): could not find signal "unrealize"
in the `GnomeCanvasEllipse' class ancestry
(gdb)

I guess I can't do it, can I? :-(

Regards,
Juan.