diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index 5154b20..c424273 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -456,7 +456,7 @@
   call delete('Xundodir', 'd')
 
   " Test undofile() with 'undodir' set to a non-existing directory.
-  call assert_equal('', undofile('Xundofoo'))
+  call assert_equal('', 'Xundofoo'->undofile())
 
   if isdirectory('/tmp')
     set undodir=/tmp