diff --git a/runtime/syntax/testdir/runtest.vim b/runtime/syntax/testdir/runtest.vim
index 9e2739f..27b7664 100644
--- a/runtime/syntax/testdir/runtest.vim
+++ b/runtime/syntax/testdir/runtest.vim
@@ -163,7 +163,7 @@
 
       call delete('done/' .. root)
 
-      call failed_tests->extend(root)
+      eval failed_tests->add(root)
       if len(failed_tests) > MAX_FAILED_COUNT
 	call Message('')
 	call Message('Too many errors, aborting')