diff --git a/libraries/AP_ADSB/AP_ADSB.cpp b/libraries/AP_ADSB/AP_ADSB.cpp
index 3040e4803f..6fd787a30a 100644
--- a/libraries/AP_ADSB/AP_ADSB.cpp
+++ b/libraries/AP_ADSB/AP_ADSB.cpp
@@ -632,7 +632,7 @@ void AP_ADSB::set_callsign(const char* str, const bool append_icao)
     } // for i
 
     if (append_icao) {
-        snprintf(&out_state.cfg.callsign[4], 5, "%04X", out_state.cfg.ICAO_id % 0x10000);
+        snprintf(&out_state.cfg.callsign[4], 5, "%04X", unsigned(out_state.cfg.ICAO_id % 0x10000));
     }
 }