diff --git a/libraries/AP_Airspeed/AP_Airspeed_SDP3X.cpp b/libraries/AP_Airspeed/AP_Airspeed_SDP3X.cpp
index 7fd8c6120b..a8e066c515 100644
--- a/libraries/AP_Airspeed/AP_Airspeed_SDP3X.cpp
+++ b/libraries/AP_Airspeed/AP_Airspeed_SDP3X.cpp
@@ -310,7 +310,7 @@ bool AP_Airspeed_SDP3X::get_temperature(float &temperature)
 /*
   check CRC for a set of bytes
  */
-bool AP_Airspeed_SDP3X::_crc(const uint8_t data[], unsigned size, uint8_t checksum)
+bool AP_Airspeed_SDP3X::_crc(const uint8_t data[], uint8_t size, uint8_t checksum)
 {
     uint8_t crc_value = 0xff;
 
diff --git a/libraries/AP_Airspeed/AP_Airspeed_SDP3X.h b/libraries/AP_Airspeed/AP_Airspeed_SDP3X.h
index 0a4f8ed808..be761335c7 100644
--- a/libraries/AP_Airspeed/AP_Airspeed_SDP3X.h
+++ b/libraries/AP_Airspeed/AP_Airspeed_SDP3X.h
@@ -49,7 +49,7 @@ public:
 private:
     void _timer();
     bool _send_command(uint16_t cmd);
-    bool _crc(const uint8_t data[], unsigned size, uint8_t checksum);
+    bool _crc(const uint8_t data[], uint8_t size, uint8_t checksum);
     float _correct_pressure(float press);
 
     float _temp;