diff --git a/test/platform/riot/coap_channel_federated_test/sender/main.c b/test/platform/riot/coap_channel_federated_test/sender/main.c
index 3964ac17..01c1c00d 100755
--- a/test/platform/riot/coap_channel_federated_test/sender/main.c
+++ b/test/platform/riot/coap_channel_federated_test/sender/main.c
@@ -8,7 +8,7 @@ typedef struct {
   char msg[512];
 } lf_msg_t;
 
-size_t serialize_msg_t(const void *user_struct, size_t user_struct_size, unsigned char *msg_buf) {
+int serialize_msg_t(const void *user_struct, size_t user_struct_size, unsigned char *msg_buf) {
   (void)user_struct_size;
   const lf_msg_t *msg = user_struct;