diff --git a/artiq/runtime/moninj.c b/artiq/runtime/moninj.c
index 67116626f..bc3bccbb6 100644
--- a/artiq/runtime/moninj.c
+++ b/artiq/runtime/moninj.c
@@ -52,7 +52,8 @@ static void moninj_monitor(const ip_addr_t *addr, u16_t port)
     reply.ttl_levels = 0;
     reply.ttl_oes = 0;
     reply.ttl_overrides = 0;
-    for(i=0;i<CONFIG_RTIO_REGULAR_TTL_COUNT;i++) {
+    /* send at most 64 channels, the protocol doesn't support more */
+    for(i=0;i<min(CONFIG_RTIO_REGULAR_TTL_COUNT, 64);i++) {
         rtio_moninj_mon_chan_sel_write(i);
         rtio_moninj_mon_probe_sel_write(0);
         rtio_moninj_mon_value_update_write(1);