Index: trunk/src/org/openstreetmap/josm/gui/widgets/HistoryComboBox.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/widgets/HistoryComboBox.java	(revision 18125)
+++ trunk/src/org/openstreetmap/josm/gui/widgets/HistoryComboBox.java	(revision 18126)
@@ -6,4 +6,5 @@
 import javax.swing.text.JTextComponent;
 
+import org.openstreetmap.josm.data.tagging.ac.AutoCompletionItem;
 import org.openstreetmap.josm.gui.tagging.ac.AutoCompletingComboBox;
 import org.openstreetmap.josm.spi.preferences.Config;
@@ -55,5 +56,12 @@
      */
     public void addCurrentItemToHistory() {
-        model.addElement((String) getEditor().getItem());
+        Object item = getEditor().getItem();
+        if (item instanceof String) {
+            model.addElement((String) item);
+        } else if (item instanceof AutoCompletionItem) {
+            model.addElement((AutoCompletionItem) item);
+        } else {
+            throw new IllegalArgumentException(item.getClass().getSimpleName() + " is not supported in addCurrentItemToHistory");
+        }
     }