diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index dec8288..ce925d8 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -269,6 +269,7 @@
   " Expand terminal options.
   call feedkeys(":set t_A\<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_equal('"set t_AB t_AF t_AL', @:)
+  call assert_fails('call feedkeys(":set <t_afoo>=\<C-A>\<CR>", "xt")', 'E474:')
 
   " Expand directories.
   call feedkeys(":set cdpath=./\<C-A>\<C-B>\"\<CR>", 'tx')