diff --git a/src/set.c b/src/set.c
index 5ed43f4..30f00ee 100644
--- a/src/set.c
+++ b/src/set.c
@@ -158,7 +158,7 @@
 }
 
 API LY_ERR
-ly_set_merge(struct ly_set *trg, struct ly_set *src, ly_bool list, void *(*duplicator)(void *obj))
+ly_set_merge(struct ly_set *trg, const struct ly_set *src, ly_bool list, void *(*duplicator)(void *obj))
 {
     uint32_t u;
     void *obj;