diff --git a/src/common/libzmqutil/mpart.c b/src/common/libzmqutil/mpart.c
index c4685edde2a4..a6ff76e5dcf1 100644
--- a/src/common/libzmqutil/mpart.c
+++ b/src/common/libzmqutil/mpart.c
@@ -102,7 +102,7 @@ static int mpart_append (zlist_t *mpart, zmq_msg_t *part)
     return 0;
 }
 
-int mpart_addmem (zlist_t *mpart, const void *buf, int size)
+int mpart_addmem (zlist_t *mpart, const void *buf, size_t size)
 {
     zmq_msg_t *part;
 
diff --git a/src/common/libzmqutil/mpart.h b/src/common/libzmqutil/mpart.h
index 253a634ee951..4adc1e505b38 100644
--- a/src/common/libzmqutil/mpart.h
+++ b/src/common/libzmqutil/mpart.h
@@ -20,7 +20,7 @@
  */
 void mpart_destroy (zlist_t *mpart);
 zlist_t *mpart_create (void);
-int mpart_addmem (zlist_t *mpart, const void *buf, int size);
+int mpart_addmem (zlist_t *mpart, const void *buf, size_t size);
 int mpart_addstr (zlist_t *mpart, const char *s);
 zlist_t *mpart_recv (void *sock);
 int mpart_send (void *sock, zlist_t *mpart);