[Gtk-sharp-list] Unhandled Exception: System.Exception: Unknown type

[Adam Tauno Williams]

Anyone got any clues given the following stack trace:
----------------------------------------------------------------
(Whitemice.Consonance.exe:6201): Gtk-CRITICAL **:
gtk_tree_model_filter_get_value: assertion `GTK_TREE_MODEL_FILTER
(model)->priv->stamp == iter->stamp' failed

Unhandled Exception: System.Exception: Unknown type 
at GLib.Value.get_Val () <0x006d0>
at Gtk.TreeModelFilter.GetValue (Gtk.TreeIter,int) <0x00068>
at Whitemice.ZOGI.GtkWidgets.TaskLists.IterTask
(Gtk.TreeModel,Gtk.TreeIter) <0x00035>
at Whitemice.ZOGI.GtkWidgets.TaskLists.RenderEnd
(Gtk.TreeViewColumn,Gtk.CellRenderer,Gtk.TreeModel,Gtk.TreeIter)
<0x00039>
at (wrapper delegate-invoke)
System.MulticastDelegate.invoke_void_TreeViewColumn_CellRenderer_TreeModel_TreeIter (Gtk.TreeViewColumn,Gtk.CellRenderer,Gtk.TreeModel,Gtk.TreeIter) <0x0005f>
at GtkSharp.TreeCellDataFuncWrapper.NativeCallback
(intptr,intptr,intptr,Gtk.TreeIter&,intptr) <0x00130>
at (wrapper native-to-managed)
GtkSharp.TreeCellDataFuncWrapper.NativeCallback
(intptr,intptr,intptr,Gtk.TreeIter&,intptr) <0x0003a>
in (unmanaged) 0xb6fe70e0
at (wrapper managed-to-native) Gtk.Application.gtk_main () <0x00004>
at Gtk.Application.Run () <0x00007>
at Whitemice.Consonance.Consonance.Main (string[]) <0x000c3>
-----------------------------------------------------------------

This TreeView occasionally dies in one of the rendering functions, not
always the same one, loading the exact same data.  Most of the time it
works, maybe 4 out of 5.

The "Unhandled Exception: System.Exception: Unknown type" doesn't mean
much to me.