diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index 91d8bfd..8a41004 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -666,7 +666,7 @@
     return "a\nb\n"
   endfunc
   command -nargs=* -complete=customlist,T1 TCmd1
-  call feedkeys(":TCmd1 \<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_fails('call feedkeys(":TCmd1 \<C-A>\<C-B>\"\<CR>", "xt")', 'E1303: Custom list completion function does not return a List but a string')
   call assert_equal('"TCmd1 ', @:)
   delcommand TCmd1
   delfunc T1
@@ -675,7 +675,7 @@
     return {}
   endfunc
   command -nargs=* -complete=customlist,T2 TCmd2
-  call feedkeys(":TCmd2 \<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_fails('call feedkeys(":TCmd2 \<C-A>\<C-B>\"\<CR>", "xt")', 'E1303: Custom list completion function does not return a List but a dict')
   call assert_equal('"TCmd2 ', @:)
   delcommand TCmd2
   delfunc T2