Index: /applications/editors/josm/plugins/comfort0/src/net/simon04/comfort0/EditLevel0LAction.java
===================================================================
--- /applications/editors/josm/plugins/comfort0/src/net/simon04/comfort0/EditLevel0LAction.java	(revision 35628)
+++ /applications/editors/josm/plugins/comfort0/src/net/simon04/comfort0/EditLevel0LAction.java	(revision 35629)
@@ -30,4 +30,5 @@
 import org.openstreetmap.josm.data.osm.PrimitiveData;
 import org.openstreetmap.josm.data.osm.RelationData;
+import org.openstreetmap.josm.data.osm.TagMap;
 import org.openstreetmap.josm.data.osm.WayData;
 import org.openstreetmap.josm.gui.MainApplication;
@@ -137,7 +138,9 @@
             newInstance.load(fromLevel0L);
 
-            final boolean equalKeys = Objects.equals(newInstance.getKeys(), fromDataSet.getKeys());
-            if (!equalKeys) {
-                final ChangePropertyCommand command = new ChangePropertyCommand(Collections.singleton(fromDataSet), newInstance.getKeys());
+            final TagMap newKeys = newInstance.getKeys();
+            if (!Objects.equals(newKeys, fromDataSet.getKeys())) {
+                fromDataSet.getKeys().keySet().forEach(key ->
+                                newKeys.computeIfAbsent(key, k -> ""));
+                final ChangePropertyCommand command = new ChangePropertyCommand(Collections.singleton(fromDataSet), newKeys);
                 commands.add(command);
             }
Index: /applications/editors/josm/plugins/comfort0/test/unit/net/simon04/comfort0/EditLevel0LActionTest.java
===================================================================
--- /applications/editors/josm/plugins/comfort0/test/unit/net/simon04/comfort0/EditLevel0LActionTest.java	(revision 35628)
+++ /applications/editors/josm/plugins/comfort0/test/unit/net/simon04/comfort0/EditLevel0LActionTest.java	(revision 35629)
@@ -69,5 +69,5 @@
         node.put("fixme", "delete me!");
         command = buildChangeCommand(dataSet, ChangePropertyCommand.class);
-        assertThat(command.getTags(), is(new TagMap("name", "Neu Broderstorf", "traffic_sign", "city_limit")));
+        assertThat(command.getTags(), is(new TagMap("name", "Neu Broderstorf", "traffic_sign", "city_limit", "fixme", "")));
         node.remove("fixme");