diff --git a/nimble/controller/src/ble_ll_hci_ev.c b/nimble/controller/src/ble_ll_hci_ev.c
index 06d370687..8d088d9da 100644
--- a/nimble/controller/src/ble_ll_hci_ev.c
+++ b/nimble/controller/src/ble_ll_hci_ev.c
@@ -618,6 +618,7 @@ ble_ll_hci_ev_send_vs_printf(uint8_t id, const char *fmt, ...)
     struct ble_hci_ev_vs *ev;
     struct ble_hci_ev *hci_ev;
     va_list ap;
+    int len;
 
     hci_ev = ble_transport_alloc_evt(1);
     if (!hci_ev) {
@@ -631,10 +632,12 @@ ble_ll_hci_ev_send_vs_printf(uint8_t id, const char *fmt, ...)
     ev->id = id;
 
     va_start(ap, fmt);
-    hci_ev->length += vsnprintf((void *)ev->data,
-                                BLE_HCI_MAX_DATA_LEN - sizeof(*ev), fmt, ap);
+    len = vsnprintf((void *)ev->data, BLE_HCI_MAX_DATA_LEN - sizeof(*ev), fmt, ap);
     va_end(ap);
 
+    ev->data[len] = 0;
+    hci_ev->length += len + 1;
+
     ble_ll_hci_event_send(hci_ev);
 }