diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index e1aff61..f945e90 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2686,6 +2686,11 @@
   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:')
+
+  call assert_fails('call extend(g:, #{foo: { -> "foo" }})', 'E704:')
+  let g:bar = 123
+  call extend(g:, #{bar: { -> "foo" }}, "keep")
+  call assert_fails('call extend(g:, #{bar: { -> "foo" }}, "force")', 'E704:')
 endfunc