diff --git a/src/session/subscription.c b/src/session/subscription.c
index 117a02507..fd057aac3 100644
--- a/src/session/subscription.c
+++ b/src/session/subscription.c
@@ -177,7 +177,7 @@ int8_t _z_trigger_subscriptions(_z_session_t *zn, const _z_keyexpr_t keyexpr, co
         s.kind = kind;
         s.timestamp = timestamp;
         _z_subscription_sptr_list_t *xs = subs;
-        _Z_DEBUG("Triggering %zd subs\n", (uintmax_t)_z_subscription_sptr_list_len(xs));
+        _Z_DEBUG("Triggering %ju subs\n", (uintmax_t)_z_subscription_sptr_list_len(xs));
         while (xs != NULL) {
             _z_subscription_sptr_t *sub = _z_subscription_sptr_list_head(xs);
             sub->ptr->_callback(&s, sub->ptr->_arg);