diff --git a/libraries/AP_CANManager/AP_SLCANIface.cpp b/libraries/AP_CANManager/AP_SLCANIface.cpp
index 62ccd0e44e..0656cece98 100644
--- a/libraries/AP_CANManager/AP_SLCANIface.cpp
+++ b/libraries/AP_CANManager/AP_SLCANIface.cpp
@@ -721,11 +721,11 @@ int16_t SLCAN::CANIface::receive(AP_HAL::CANFrame& out_frame, uint64_t& rx_time,
         uint32_t num_bytes = _port->available_locked(_serial_lock_key);
         // flush bytes from port
         while (num_bytes--) {
-            int16_t ret = _port->read_locked(_serial_lock_key);
-            if (ret < 0) {
+            uint8_t b;
+            if (!_port->read_locked(_serial_lock_key, b)) {
                 break;
             }
-            addByte(ret);
+            addByte(b);
             if (!rx_queue_.space()) {
                 break;
             }