Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/ExpressionFactory.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/ExpressionFactory.java	(revision 17762)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/ExpressionFactory.java	(revision 17764)
@@ -207,5 +207,5 @@
         FACTORY_MAP.put("print", Factory.of(Object.class, Functions::print));
         FACTORY_MAP.put("println", Factory.of(Object.class, Functions::println));
-        FACTORY_MAP.put("prop", Factory.ofEnv(String.class, Functions::prop));
+        FACTORY_MAP.put("prop", Factory.ofEnv(String.class, String.class, Functions::prop, Functions::prop));
         FACTORY_MAP.put("red", Factory.of(Color.class, Functions::red));
         FACTORY_MAP.put("regexp_match", Factory.of(String.class, String.class, String.class, Functions::regexp_match, Functions::regexp_match));