diff --git a/src/testdir/test_assign.vim b/src/testdir/test_assign.vim
index efc9d08..317d458 100644
--- a/src/testdir/test_assign.vim
+++ b/src/testdir/test_assign.vim
@@ -29,3 +29,17 @@
   let &t_xx = ""
   call assert_fails('let x = &t_xx', 'E15')
 endfunc
+
+func Test_let_option_error()
+  let _w = &tw
+  let &tw = 80
+  call assert_fails('let &tw .= 1', 'E734')
+  call assert_equal(80, &tw)
+  let &tw = _w
+
+  let _w = &fillchars
+  let &fillchars = "vert:|"
+  call assert_fails('let &fillchars += "diff:-"', 'E734')
+  call assert_equal("vert:|", &fillchars)
+  let &fillchars = _w
+endfunc