src/jdk/nashorn/internal/objects/NativeArguments.java

@@ -123,11 +123,11 @@
      * setArgument is used for named argument set.
      */
     @Override
     public void setArgument(final int key, final Object value) {
         if (namedArgs.has(key)) {
-            namedArgs.set(key, value, false);
+            namedArgs = namedArgs.set(key, value, false);
         }
     }
 
     @Override
     public int getInt(final Object key) {