diff --git a/executables/nr-gnb.c b/executables/nr-gnb.c
index b45f41c82a..143f105847 100644
--- a/executables/nr-gnb.c
+++ b/executables/nr-gnb.c
@@ -117,7 +117,10 @@ static void tx_func(void *param)
   int slot_rx = info->slot_rx;
   int absslot_tx = info->timestamp_tx / info->gNB->frame_parms.get_samples_per_slot(slot_tx, &info->gNB->frame_parms);
   int absslot_rx = absslot_tx - info->gNB->RU_list[0]->sl_ahead;
-
+  if (absslot_rx < 0) {
+    LOG_W(NR_PHY, "Slot ahead %d is larger than absslot_tx %d. Cannot start TX yet.\n", info->gNB->RU_list[0]->sl_ahead, absslot_tx);
+    return;
+  }
   LOG_D(NR_PHY, "%d.%d running tx_func\n", frame_tx, slot_tx);
   PHY_VARS_gNB *gNB = info->gNB;
   module_id_t module_id = gNB->Mod_id;