diff --git a/src/msgpack/sbuffer.h b/src/msgpack/sbuffer.h
index 778dea71..4ade093b 100644
--- a/src/msgpack/sbuffer.h
+++ b/src/msgpack/sbuffer.h
@@ -64,7 +64,7 @@ static inline void msgpack_sbuffer_free(msgpack_sbuffer* sbuf)
 #define MSGPACK_SBUFFER_INIT_SIZE 8192
 #endif
 
-static inline int msgpack_sbuffer_write(void* data, const char* buf, unsigned int len)
+static inline int msgpack_sbuffer_write(void* data, const char* buf, size_t len)
 {
 	msgpack_sbuffer* sbuf = (msgpack_sbuffer*)data;
 
diff --git a/src/msgpack/sbuffer.hpp b/src/msgpack/sbuffer.hpp
index 14c5d2a2..fce56ad5 100644
--- a/src/msgpack/sbuffer.hpp
+++ b/src/msgpack/sbuffer.hpp
@@ -47,7 +47,7 @@ public:
 	}
 
 public:
-	void write(const char* buf, unsigned int len)
+	void write(const char* buf, size_t len)
 	{
 		if(base::alloc - base::size < len) {
 			expand_buffer(len);
diff --git a/src/msgpack/zbuffer.h b/src/msgpack/zbuffer.h
index efdd3049..c5630514 100644
--- a/src/msgpack/zbuffer.h
+++ b/src/msgpack/zbuffer.h
@@ -65,7 +65,7 @@ static inline char* msgpack_zbuffer_release_buffer(msgpack_zbuffer* zbuf);
 #define MSGPACK_ZBUFFER_RESERVE_SIZE 512
 #endif
 
-static inline int msgpack_zbuffer_write(void* data, const char* buf, unsigned int len);
+static inline int msgpack_zbuffer_write(void* data, const char* buf, size_t len);
 
 static inline bool msgpack_zbuffer_expand(msgpack_zbuffer* zbuf);
 
@@ -123,7 +123,7 @@ bool msgpack_zbuffer_expand(msgpack_zbuffer* zbuf)
 	return true;
 }
 
-int msgpack_zbuffer_write(void* data, const char* buf, unsigned int len)
+int msgpack_zbuffer_write(void* data, const char* buf, size_t len)
 {
 	msgpack_zbuffer* zbuf = (msgpack_zbuffer*)data;
 
diff --git a/src/msgpack/zbuffer.hpp b/src/msgpack/zbuffer.hpp
index 4ccb487b..69e2eb79 100644
--- a/src/msgpack/zbuffer.hpp
+++ b/src/msgpack/zbuffer.hpp
@@ -40,7 +40,7 @@ public:
 	}
 
 public:
-	void write(const char* buf, unsigned int len)
+	void write(const char* buf, size_t len)
 	{
 		if(msgpack_zbuffer_write(this, buf, len) < 0) {
 			throw std::bad_alloc();