diff --git a/notes.c b/notes.c
index fa7cdf7dd2..b69c0b8257 100644
--- a/notes.c
+++ b/notes.c
@@ -927,8 +927,12 @@ static int string_list_add_one_ref(const char *refname, const unsigned char *sha
 	return 0;
 }
 
+/*
+ * The list argument must have strdup_strings set on it.
+ */
 void string_list_add_refs_by_glob(struct string_list *list, const char *glob)
 {
+	assert(list->strdup_strings);
 	if (has_glob_specials(glob)) {
 		for_each_glob_ref(string_list_add_one_ref, glob, list);
 	} else {