diff --git a/include/margo.h b/include/margo.h
index 39fa8029d52826419b5a3cd239568b093d419b35..f3666a2c14c9c0d295e0a01c76afbf90f069dc59 100644
--- a/include/margo.h
+++ b/include/margo.h
@@ -20,6 +20,7 @@ struct margo_instance;
 typedef struct margo_instance* margo_instance_id;
 
 #define MARGO_INSTANCE_NULL ((margo_instance_id)NULL)
+#define MARGO_DEFAULT_MPLEX_ID 0
 
 
 /**
diff --git a/src/margo.c b/src/margo.c
index e36a2bf39566f5540cde6ef007d859332a49d0c1..86f4fa277b7036852ebe1a229078d211934bd620 100644
--- a/src/margo.c
+++ b/src/margo.c
@@ -833,9 +833,9 @@ int margo_register_mplex(margo_instance_id mid, hg_id_t id, uint32_t mplex_id, A
     struct mplex_key key;
     struct mplex_element *element;
 
-    /* mplex_id can't be zero; that's the default handler pool */
-    if(!mplex_id)
-        return(-1);
+    /* nothing to do, we'll let the handler pool take this directly */
+    if(mplex_id == MARGO_DEFAULT_MPLEX_ID)
+        return(0);
 
     memset(&key, 0, sizeof(key));
     key.id = id;