diff --git a/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.cpp b/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.cpp
index ab8b6680cc..ee8f48e54f 100644
--- a/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.cpp
+++ b/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.cpp
@@ -475,7 +475,7 @@ uint32_t AP_Frsky_SPort_Passthrough::calc_batt(uint8_t instance)
  * true if we need to respond to the last polling byte
  * for FrSky SPort Passthrough (OpenTX) protocol (X-receivers)
  */
-bool AP_Frsky_SPort_Passthrough::is_passthrough_byte(const uint8_t byte)
+bool AP_Frsky_SPort_Passthrough::is_passthrough_byte(const uint8_t byte) const
 {
 #if HAL_WITH_FRSKY_TELEM_BIDIRECTIONAL
     if( byte == _SPort_bidir.downlink1_sensor_id || byte == _SPort_bidir.downlink2_sensor_id ) {
diff --git a/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.h b/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.h
index 22bed7f815..62f20ba256 100644
--- a/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.h
+++ b/libraries/AP_Frsky_Telem/AP_Frsky_SPort_Passthrough.h
@@ -146,7 +146,7 @@ private:
     void send_sport_frame(uint8_t frame, uint16_t appid, uint32_t data);
 
     // true if we need to respond to the last polling byte
-    bool is_passthrough_byte(const uint8_t byte);
+    bool is_passthrough_byte(const uint8_t byte) const;
 
     uint8_t _paramID;