diff --git a/libuavcan_drivers/stm32/driver/include/uavcan_stm32/can.hpp b/libuavcan_drivers/stm32/driver/include/uavcan_stm32/can.hpp
index 6cd61280e8..196b9490e3 100644
--- a/libuavcan_drivers/stm32/driver/include/uavcan_stm32/can.hpp
+++ b/libuavcan_drivers/stm32/driver/include/uavcan_stm32/can.hpp
@@ -217,7 +217,7 @@ public:
      * Range is [1, 3].
      * Value of 3 suggests that priority inversion could be taking place.
      */
-    uavcan::uint8_t getPeakNumTxMailboxesUsed() const { return peak_tx_mailbox_index_ + 1; }
+    uavcan::uint8_t getPeakNumTxMailboxesUsed() const { return uavcan::uint8_t(peak_tx_mailbox_index_ + 1); }
 };
 
 /**