Index: /trunk/src/org/openstreetmap/josm/command/Command.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/command/Command.java	(revision 17225)
+++ /trunk/src/org/openstreetmap/josm/command/Command.java	(revision 17226)
@@ -4,5 +4,5 @@
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.HashMap;
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -20,4 +20,5 @@
 import org.openstreetmap.josm.data.osm.visitor.OsmPrimitiveVisitor;
 import org.openstreetmap.josm.tools.CheckParameterUtil;
+import org.openstreetmap.josm.tools.Utils;
 
 /**
@@ -123,5 +124,5 @@
 
     /** the map of OsmPrimitives in the original state to OsmPrimitives in cloned state */
-    private Map<OsmPrimitive, PrimitiveData> cloneMap = new HashMap<>();
+    private Map<OsmPrimitive, PrimitiveData> cloneMap = Collections.emptyMap();
 
     /** the dataset which this command is applied to */
@@ -154,5 +155,5 @@
             osm.accept(visitor);
         }
-        cloneMap = visitor.orig;
+        cloneMap = Utils.toUnmodifiableMap(visitor.orig);
         return true;
     }