diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index b2ef0db..56c7b99 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -686,6 +686,19 @@
     X()
   END
   v9.CheckSourceFailure(lines, 'E1324: Using an Object as a String', 2)
+
+  # Use a null object variable that vim wants to force to number.
+  lines =<< trim END
+    vim9script
+
+    def X()
+      var o = null_object
+      var l = [ 1, o]
+      sort(l, 'N')
+    enddef
+    X()
+  END
+  v9.CheckSourceFailure(lines, 'E1324: Using an Object as a String', 3)
 enddef
 
 " Null object assignment and comparison