diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index beabec8..05c3949 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -74,6 +74,7 @@
 
   call assert_fails('echo jsonencode(function("tr"))', 'E474:')
   call assert_fails('echo jsonencode([function("tr")])', 'E474:')
+  call assert_fails('echo jsonencode({"key":v:none})', 'E474:')
 endfunc
 
 func Test_decode()