diff --git a/src/testdir/test_spell.vim b/src/testdir/test_spell.vim
index e6c3729..0688d61 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -18,3 +18,12 @@
   bwipe!
   set nospell
 endfunc
+
+func Test_z_equal_on_invalid_utf8_word()
+  split
+  set spell
+  call setline(1, "\xff")
+  norm z=
+  set nospell
+  bwipe!
+endfunc