diff --git a/src/libknot/rrtype/opt.c b/src/libknot/rrtype/opt.c
index 9f2a9b878dc59a05c9b67d6e54348c446e68fffb..b24e06f2609c26718f365fecce07b1297294b142 100644
--- a/src/libknot/rrtype/opt.c
+++ b/src/libknot/rrtype/opt.c
@@ -659,7 +659,7 @@ int knot_edns_client_subnet_write(uint8_t *option, size_t option_len,
 
 _public_
 int knot_edns_client_subnet_parse(knot_edns_client_subnet_t *ecs,
-                                  const uint8_t *option, size_t option_len)
+                                  const uint8_t *option, uint16_t option_len)
 {
 	if (ecs == NULL || option == NULL) {
 		return KNOT_EINVAL;
diff --git a/src/libknot/rrtype/opt.h b/src/libknot/rrtype/opt.h
index c5b8861f7e54942110c482838eccfcc92ad84893..bd0bc70acfa2cb9dd7be18807bda54658751de71 100644
--- a/src/libknot/rrtype/opt.h
+++ b/src/libknot/rrtype/opt.h
@@ -428,7 +428,7 @@ int knot_edns_client_subnet_write(uint8_t *option, size_t option_len,
  * \return Error code, KNOT_EOK if successful.
  */
 int knot_edns_client_subnet_parse(knot_edns_client_subnet_t *ecs,
-                                  const uint8_t *option, size_t option_len);
+                                  const uint8_t *option, uint16_t option_len);
 
 /*!
  * \brief Set address to the ECS structure.