diff --git a/src/testdir/test_startup.vim b/src/testdir/test_startup.vim
index 41bb12f..6b678fb 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -157,6 +157,7 @@
 " horizontally or vertically.
 func Test_o_arg()
   let after =<< trim [CODE]
+    set cpo&vim
     call writefile([winnr("$"),
 		\ winheight(1), winheight(2), &lines,
 		\ winwidth(1), winwidth(2), &columns,