diff --git a/src/msgpack/pack.h b/src/msgpack/pack.h
index f7af3b4f..2502bd6a 100644
--- a/src/msgpack/pack.h
+++ b/src/msgpack/pack.h
@@ -40,7 +40,7 @@ extern "C" {
  * @{
  */
 
-typedef int (*msgpack_packer_write)(void* data, const char* buf, unsigned int len);
+typedef int (*msgpack_packer_write)(void* data, const char* buf, size_t len);
 
 typedef struct msgpack_packer {
 	void* data;