diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 24bdc42..a7621f5 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -701,6 +701,10 @@
   " Test empty option
   set diffopt=
   call assert_equal([], getcompletion('set diffopt-=', 'cmdline'))
+  " Test all possible values
+  call assert_equal(['filler', 'context:', 'iblank', 'icase', 'iwhite', 'iwhiteall', 'iwhiteeol', 'horizontal',
+        \ 'vertical', 'closeoff', 'hiddenoff', 'foldcolumn:', 'followwrap', 'internal', 'indent-heuristic', 'algorithm:', 'linematch:'],
+        \ getcompletion('set diffopt=', 'cmdline'))
   set diffopt&
 
   " Test escaping output