Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Functions.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Functions.java	(revision 15721)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Functions.java	(revision 15723)
@@ -11,4 +11,5 @@
 import java.util.Locale;
 import java.util.Map.Entry;
+import java.util.Objects;
 import java.util.TreeSet;
 import java.util.regex.Matcher;
@@ -311,5 +312,8 @@
     @NullableArguments
     public static String concat(Object... args) { // NO_UCD (unused code)
-        return Arrays.stream(args).map(String::valueOf).collect(Collectors.joining());
+        return Arrays.stream(args)
+                .filter(Objects::nonNull)
+                .map(String::valueOf)
+                .collect(Collectors.joining());
     }