Index: src/org/openstreetmap/josm/gui/dialogs/PropertiesDialog.java
===================================================================
--- src/org/openstreetmap/josm/gui/dialogs/PropertiesDialog.java	(revision 247)
+++ src/org/openstreetmap/josm/gui/dialogs/PropertiesDialog.java	(revision 248)
@@ -208,7 +208,8 @@
 			public void actionPerformed(ActionEvent e) {
 				String key = keys.getEditor().getItem().toString();
-				Vector<String> newValues = new Vector<String>(allData.get(key));
-	            values.setModel(new DefaultComboBoxModel(newValues));
-	            
+				if (allData.containsKey(key)) {
+					Vector<String> newValues = new Vector<String>(allData.get(key));
+					values.setModel(new DefaultComboBoxModel(newValues));
+				}
             }