diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 189ff4d..1b56df5 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -236,6 +236,18 @@
       assert_equal(#{one: 1}, d)
   END
   CheckScriptSuccess(lines)
+
+  # appending to NULL list from a function
+  lines =<< trim END
+      vim9script
+      var list: list<string>
+      def Func()
+        list += ['a', 'b']
+      enddef
+      Func()
+      assert_equal(['a', 'b'], list)
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def Test_single_letter_vars()