diff --git a/include/zmq.h b/include/zmq.h
index 23c6bee0..f2f36c2b 100644
--- a/include/zmq.h
+++ b/include/zmq.h
@@ -165,7 +165,7 @@ ZMQ_EXPORT int zmq_msg_close (zmq_msg_t *msg);
 ZMQ_EXPORT int zmq_msg_move (zmq_msg_t *dest, zmq_msg_t *src);
 ZMQ_EXPORT int zmq_msg_copy (zmq_msg_t *dest, zmq_msg_t *src);
 ZMQ_EXPORT void *zmq_msg_data (zmq_msg_t *msg);
-ZMQ_EXPORT size_t zmq_msg_size (zmq_msg_t *msg);
+ZMQ_EXPORT ssize_t zmq_msg_size (zmq_msg_t *msg);
 ZMQ_EXPORT int zmq_getmsgopt (zmq_msg_t *msg, int option, void *optval,
     size_t *optvallen);
 
diff --git a/src/zmq.cpp b/src/zmq.cpp
index a0ee5655..3d3e70a1 100644
--- a/src/zmq.cpp
+++ b/src/zmq.cpp
@@ -572,7 +572,7 @@ void *zmq_msg_data (zmq_msg_t *msg_)
     return ((zmq::msg_t*) msg_)->data ();
 }
 
-size_t zmq_msg_size (zmq_msg_t *msg_)
+ssize_t zmq_msg_size (zmq_msg_t *msg_)
 {
     if (!msg_) {
         errno = EFAULT;