diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 4337606..e1aff61 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2681,4 +2681,12 @@
   call assert_fails('call gettext(1)', 'E475:')
 endfunc
 
+func Test_builtin_check()
+  call assert_fails('let g:["trim"] = {x -> " " .. x}', 'E704:')
+  call assert_fails('let g:.trim = {x -> " " .. x}', 'E704:')
+  call assert_fails('let s:["trim"] = {x -> " " .. x}', 'E704:')
+  call assert_fails('let s:.trim = {x -> " " .. x}', 'E704:')
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab