diff --git a/src/or/parsecommon.c b/src/or/parsecommon.c
index 6c3dd3100e..9bd00e17ce 100644
--- a/src/or/parsecommon.c
+++ b/src/or/parsecommon.c
@@ -426,7 +426,7 @@ find_by_keyword_(smartlist_t *s, directory_keyword keyword,
  * NULL if no such keyword is found.
  */
 directory_token_t *
-find_opt_by_keyword(smartlist_t *s, directory_keyword keyword)
+find_opt_by_keyword(const smartlist_t *s, directory_keyword keyword)
 {
   SMARTLIST_FOREACH(s, directory_token_t *, t, if (t->tp == keyword) return t);
   return NULL;
diff --git a/src/or/parsecommon.h b/src/or/parsecommon.h
index 903d94478b..d33faf8ec7 100644
--- a/src/or/parsecommon.h
+++ b/src/or/parsecommon.h
@@ -314,7 +314,7 @@ directory_token_t *find_by_keyword_(smartlist_t *s,
 #define find_by_keyword(s, keyword) \
   find_by_keyword_((s), (keyword), #keyword)
 
-directory_token_t *find_opt_by_keyword(smartlist_t *s,
+directory_token_t *find_opt_by_keyword(const smartlist_t *s,
                                        directory_keyword keyword);
 smartlist_t * find_all_by_keyword(const smartlist_t *s, directory_keyword k);