Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/ExpressionFactory.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/ExpressionFactory.java	(revision 17830)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/ExpressionFactory.java	(revision 17832)
@@ -60,5 +60,6 @@
                     return identity;
                 } else if (args.size() == 1) {
-                    return unaryOperator.applyAsDouble(Cascade.convertTo(args.get(0).evaluate(env), Double.class));
+                    Double arg = Cascade.convertTo(args.get(0).evaluate(env), Double.class);
+                    return arg == null ? null : unaryOperator.applyAsDouble(arg);
                 } else {
                     return args.stream()