--- a/src/CellBuffer.h
+++ b/src/CellBuffer.h
@@ -98,6 +98,9 @@
 
 	void EnsureUndoRoom();
 
+	// Private so UndoHistory objects can not be copied
+	UndoHistory(const UndoHistory &);
+
 public:
 	UndoHistory();
 	~UndoHistory();