diff --git a/libraries/RC_Channel/RC_Channel.cpp b/libraries/RC_Channel/RC_Channel.cpp
index 1d98e49f02..4d4e69273a 100644
--- a/libraries/RC_Channel/RC_Channel.cpp
+++ b/libraries/RC_Channel/RC_Channel.cpp
@@ -450,9 +450,9 @@ void RC_Channel::init_aux_function(const aux_func_t ch_option, const aux_switch_
         do_aux_function(ch_option, ch_flag);
         break;
     default:
-        gcs().send_text(MAV_SEVERITY_WARNING, "Failed to initialise RC function (%u)", ch_option);
+        gcs().send_text(MAV_SEVERITY_WARNING, "Failed to initialise RC function (%u)", (unsigned)ch_option);
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
-        AP_HAL::panic("RC function (%u) initialisation not handled", ch_option);
+        AP_HAL::panic("RC function (%u) initialisation not handled", (unsigned)ch_option);
 #endif
         break;
     }