diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 24a7f95..01f29d9 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -371,10 +371,17 @@
   assert_fails('bufname([])', 'E1220:')
 enddef
 
+let s:bufnr_res = 0
+
 def Test_bufnr()
   var buf = bufnr()
   bufnr('%')->assert_equal(buf)
 
+  # check the lock is not taken over through the stack
+  const nr = 10
+  bufnr_res = bufnr()
+  bufnr_res = 12345
+
   buf = bufnr('Xdummy', true)
   buf->assert_notequal(-1)
   exe 'bwipe! ' .. buf