diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 0d7269b..fe19a16 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -7,5 +7,8 @@
 tabbed_indent_required_check: false
 signoff_check: true
 
+# Make sure cos_patch trailer is present.
+cos_patch_trailer_check: true
+
 # Make sure RELEASE_NOTE field is present.
 release_note_field_check: true