Index: trunk/src/org/openstreetmap/josm/gui/dialogs/changeset/ChangesetCacheTableRowSorter.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/changeset/ChangesetCacheTableRowSorter.java	(revision 19510)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/changeset/ChangesetCacheTableRowSorter.java	(revision 19512)
@@ -35,5 +35,5 @@
 
         // column 4 - Created at
-        setComparator(4, comparing(Changeset::getCreatedAt));
+        setComparator(4, comparing(Changeset::getCreatedAt, nullsLast(naturalOrder())));
 
         // column 5 - Closed at