diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 831a894..131db10 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -939,9 +939,13 @@
 func Test_append()
   enew!
   split
-  call append(0, ["foo"])
-  call append(1, [])
-  call append(1, test_null_list())
+  call assert_equal(0, append(1, []))
+  call assert_equal(0, append(1, test_null_list()))
+  call assert_equal(0, append(0, ["foo"]))
+  call assert_equal(0, append(1, []))
+  call assert_equal(0, append(1, test_null_list()))
+  call assert_equal(0, append(8, []))
+  call assert_equal(0, append(9, test_null_list()))
   call assert_equal(['foo', ''], getline(1, '$'))
   split
   only