diff --git a/libraries/AP_NavEKF/AP_NavEKF.cpp b/libraries/AP_NavEKF/AP_NavEKF.cpp
index c711d24b7b..6987224f2b 100644
--- a/libraries/AP_NavEKF/AP_NavEKF.cpp
+++ b/libraries/AP_NavEKF/AP_NavEKF.cpp
@@ -728,7 +728,7 @@ bool NavEKF::use_compass(void) const
 // The sign convention is that a RH physical rotation of the sensor about an axis produces both a positive flow and gyro rate
 // msecFlowMeas is the scheduler time in msec when the optical flow data was received from the sensor.
 // NOTE: AP_NavEKF does not use the posOffset data
-void NavEKF::writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, Vector3f &posOffset)
+void NavEKF::writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, const Vector3f &posOffset)
 {
     if (core) {
         core->writeOptFlowMeas(rawFlowQuality, rawFlowRates, rawGyroRates, msecFlowMeas);
diff --git a/libraries/AP_NavEKF/AP_NavEKF.h b/libraries/AP_NavEKF/AP_NavEKF.h
index ac8350369a..8b80890c16 100644
--- a/libraries/AP_NavEKF/AP_NavEKF.h
+++ b/libraries/AP_NavEKF/AP_NavEKF.h
@@ -181,7 +181,7 @@ public:
     // rawGyroRates are the sensor rotation rates in rad/sec measured by the sensors internal gyro
     // The sign convention is that a RH physical rotation of the sensor about an axis produces both a positive flow and gyro rate
     // msecFlowMeas is the scheduler time in msec when the optical flow data was received from the sensor.
-    void  writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, Vector3f &posOffset);
+    void  writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, const Vector3f &posOffset);
 
     // return data for debugging optical flow fusion
     void getFlowDebug(float &varFlow, float &gndOffset, float &flowInnovX, float &flowInnovY, float &auxInnov, float &HAGL, float &rngInnov, float &range, float &gndOffsetErr) const;