diff --git a/contrib/include/wx/gizmos/splittree.h b/contrib/include/wx/gizmos/splittree.h
index d5e178bb59..5758dde534 100644
--- a/contrib/include/wx/gizmos/splittree.h
+++ b/contrib/include/wx/gizmos/splittree.h
@@ -84,7 +84,7 @@ public:
 	// Calculate the tree overall size so we can set the scrollbar
 	// correctly
 	void CalcTreeSize(wxRect& rect);
-	void CalcTreeSize(wxTreeItemId& id, wxRect& rect);
+	void CalcTreeSize(const wxTreeItemId& id, wxRect& rect);
 
 	// Adjust the containing wxScrolledWindow's scrollbars appropriately
 	void AdjustRemoteScrollbars();
diff --git a/contrib/src/gizmos/splittree.cpp b/contrib/src/gizmos/splittree.cpp
index 8dbd9221e4..76cd79471c 100644
--- a/contrib/src/gizmos/splittree.cpp
+++ b/contrib/src/gizmos/splittree.cpp
@@ -277,7 +277,7 @@ void wxRemotelyScrolledTreeCtrl::CalcTreeSize(wxRect& rect)
 	CalcTreeSize(GetRootItem(), rect);
 }
 
-void wxRemotelyScrolledTreeCtrl::CalcTreeSize(wxTreeItemId& id, wxRect& rect)
+void wxRemotelyScrolledTreeCtrl::CalcTreeSize(const wxTreeItemId& id, wxRect& rect)
 {
 	// TODO: implement GetFirst/NextVisibleItem
 	// for wxGenericTreeCtrl, plus GetBoundingRect.