Index: trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 13194)
+++ trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 13195)
@@ -24,4 +24,5 @@
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 import java.util.function.Predicate;
@@ -311,5 +312,5 @@
                     try {
                         final String val = ai.val instanceof Expression
-                                ? (String) ((Expression) ai.val).evaluate(new Environment())
+                                ? Optional.of(((Expression) ai.val).evaluate(new Environment())).map(Object::toString).orElse(null)
                                 : ai.val instanceof String
                                 ? (String) ai.val