diff --git a/libraries/AP_CANManager/AP_MAVLinkCAN.cpp b/libraries/AP_CANManager/AP_MAVLinkCAN.cpp
index 12dfd5879db..ecf1cdb60fe 100644
--- a/libraries/AP_CANManager/AP_MAVLinkCAN.cpp
+++ b/libraries/AP_CANManager/AP_MAVLinkCAN.cpp
@@ -207,7 +207,7 @@ void AP_MAVLinkCAN::_handle_can_filter_modify(const mavlink_message_t &msg)
     mavlink_can_filter_modify_t p;
     mavlink_msg_can_filter_modify_decode(&msg, &p);
     const int8_t bus = int8_t(p.bus)-1;
-    if (bus >= HAL_NUM_CAN_IFACES || hal.can[bus] == nullptr) {
+    if (bus < 0 || bus >= HAL_NUM_CAN_IFACES || hal.can[bus] == nullptr) {
         return;
     }
     if (p.num_ids > ARRAY_SIZE(p.ids)) {