Index: /trunk/src/org/openstreetmap/josm/gui/mappaint/MapPaintMenu.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/mappaint/MapPaintMenu.java	(revision 18353)
+++ /trunk/src/org/openstreetmap/josm/gui/mappaint/MapPaintMenu.java	(revision 18354)
@@ -46,5 +46,8 @@
 
         private void toggleStyle() {
-            MapPaintStyles.toggleStyleActive(MapPaintStyles.getStyles().getStyleSources().indexOf(style));
+            int index = MapPaintStyles.getStyles().getStyleSources().indexOf(style);
+            if (index >= 0) {
+                MapPaintStyles.toggleStyleActive(index);
+            }
             updateButton();
         }